This is true for the pure fact that I believe some of the core classes (one specifically) was left out in regards to the new stand alone games.
My thesis: There are actually 5 different "core" professions in the world of PvP, compared to the 6 Anet has told us based off how the Primary attributes work for each profession.
What I will do: I will briefly dive into each profession from Prophecies(and soon possibly updated to Factions and Nightfall) and along the way attempt to determine what "core" is. At the moment we all believe it to be (for the majority) the broadest with no specific direction. Keeping this in mind while I attempt to hit the functions of what all 6(possibly 10 if I get back online later

Prophecies The first of all campaigns, and the one with the named "core" professions that would be compatible and competible in all future environments henceforth. Being very basic in skill selection and great in terms of power no skill from Prophecies should ever be underestimated to this day in most PvP formats. This does not mean every skill is a chuck norris round house kick but creativity had definitely flown through the design team, as many failures of Prophecies skills there are twice as many successors used in PvP formats today.
Warrior
By far the most popular class in the PvE world, and the best form of General pressure in PvP. This high armor class has a great skill selection from ranging on pure damage pressure, to condition depending on weapon and skill selection. The warrior is definitely only focused when the skill bar is accustomed to do so. Warriors Primary Attribute reads:
"When you use attack skills, each point of Strength gives you 1% armor penetration. Many skills, especially those related to surviving and inflicting damage, become more effective with higher Strength." -Guildwiki.org
The warrior is obviously a master at all weapons, ranged or melee. Dealing more damage per blow then other classes on average. So what makes the warriors "broad"? Starting off, how its primary attribute does not directly affect the other attributes, therefore not limiting strength to say a wand. As innefective as that is perhaps one day we could even see a change in that, but as it is strength is limiting only to weapons. So since he only deals more damage with weapons, althought ANY weapon is something I would consider "broad" and therefore "core".
Result: Core Class
Ranger
Expertise: "For each rank of Expertise, the Energy cost of all of your attacks, Rituals, touch skills, and Rangers skills are decreased by 4%. Several skills, especially those related to Energy costs and skill recharge times, become more effective with higher Expertise." (sic)
In otherwords the ranger primarily reduces the energy cost of every non- spell skill. This is commonly debated as the most powerful primary attribute in PvP alongside with Soul Reaping(we will get to this later).
As powerful as it is, should the ranger be considered "core"? I think the answer is intuitively obvious and therefore yes. The ranger has a wide variety in the skills he can select to become more energy efficient for himself compared to the other classes. Example Touch Ranger.
Result: Core Class
Mesmer
Fast casting reduces the amount of time required for casting spells
http://gw.gamewikis.org/wiki/Fast_Casting
Obviously after reading through that summary quite quickly you can notice that the mesmer has great benefits to its fast casting primary attribute.
This works for "all spells", this cannot be as general as possible. Everything from Meteor Shower to Reversal of Fortune gain benefit from fast casting.
Result: Core Class
Elementalist
"For each rank of Energy Storage, your maximum energy increases by 3. Several skills, related to gaining Health or Energy, become more effective with a higher Energy Storage."
This is a very obvious and general attribute as well, the ability to use energy for any purpose required in consecutive order is powerful in its own. The ability to use Ranger Bow attacks, no matter what the cost, again as ineffective as it may be, is still possible. Healing costs too high? Elementalist. Want that extra monk support? Elementalist. Looking for a self sustaining build? More then likely.. Elementalist. Ether Prodigy, has fueled this profession in PvP since the beginning of competitive GvG, and is still used today for its flexibility whether it is a monk or ritualist secondary to help in the monk backlines, Assasin secondary to help in the split tactics or gank etc etc.
This is by far a core profession.
Result: Core Class
Necromancer
Soul reaping reads. For each point of Soul Reaping, you gain 1 Energy whenever a creature near you dies. You gain half that amount for Spirits.
This one is difficult to evaluate although it is not completely core as in the other professions, it is truly not specific yet it is something else: conditional.
In other words something must be in effect for this effect to work. The ability specifically says something needs to die in order to gain benefit. Compared to the other professions we have seen this is truly a different turn of events. In the PvE atmosphere it is obvious to say "of course this is a core profession things die all the time!" Yet in PvP it is theorhetically possible that nothing dies until the last minute, yet. Something must die in order for the game to end in any PvP atmosphere essentially, especially relating to GvG and most HA maps. Since something must happen and this is affected by the "must" and it is not specific as to what or what spells and skills, as much as I am hesitant this profession is considered core.
Result: Core Class
Monk
Divine Favor reads:
"For each rank of Divine Favor, allies are healed for 3.2 whenever you cast Monk spells on them. Several Monk skills, especially spells relating to energy gain and healing, become more effective with a higher Divine Favor."
This is where the cruel turn of fate turns towards. The monk has a very huge fatal in it to ever be considered "broad'/"core". It restricts its primary attribute to its other skill branches. There is no reason whatsoever in the PvP competitive atmosphere to change a monks secondary profession other then the pure selection of how to handle energy management. Example, a Necro/Monk could profit from soul reaping and healing spells, yet a Monk/Necro gains close to absolutely nothing. Not one necro skill is aided by divine favor. This applies to every Warrior, Ranger, Mesmer, Elementalist skill as well. The monk is not "broad" whatsoever, it only profits from using its own tree skill compared to the other five in which we have seen to be able to handle skills from different professions. The monk is sadly....
Result: Specified Class
So what does this mean? Did Anet drop the bomb? Why is there only 5 core professions in the core campaign? Stop worrying first off, this is observation I have done and evaluation of what the definition "core" should actually determine. If Anet still considers these professions core, then it is obviously only on the basis of the fact that "we got here first" compared to "we are the most flexible". Which in more then one time Anet has thought it was the second. This is sort of a head twister I agree but it comes clearer upon more and more consideration. Why should the other 5 get PA's that work with other professions compared to the monk? What makes him different? Well too much honestly, he can only aid in his own skills and not benefit from any other secondary profession in any different manner. I suggest a reworking of Divine Favor entirely, to me from a personal flavor standpoint Divine Favor is not something that should relate to monk skills but spells on allies and enemies. Not any accordance to religion but if "heaven" is on your side, then you should be able to "banish" your foes with more power, and "aid" your allies with whatever is given to you.
A proposed reworking of Divine Favor: "For each rank of Divine Favor, allies are healed for 3.2 whenever you cast spells on them, also any spells cast against enemies deal 3.2 more damage to them. Several Monk skills, especially spells relating to smiting, and healing, become more effective with a higher Divine Favor."
THIS WAS NOT TESTED, JUST A PROPOSED IDEA TO CONSIDER THIS CLASS CORE.
If a similar plan such as this was implemented, we would definitely have a more broad profession ahead of us. Being able to use mesmer remove hex skills with benefit, or elementalist damaging spells with more power. The monk profession itself would have very distinctive qualities compared to the other professions and not just diving into its own skill sets.