It's kind of confusing because, well, you're still
whacking on them, but ...
Actually, standard warrior armor is AL80 + AL20 vs. physical - basically this means AL 80 vs elemental and AL100 vs physical. So, yes, an elemental weapon will do more damage against the warrior because you're going against the AL80 instead of the AL100, as you would be if using slashing/blunt/piercing (physical).
On the other hand, standard ranger armor is AL70 + AL30 vs elemental so slashing would do more damage than elemental.
Standard elementalist armor is AL60 + AL10 vs elemental + AL10 vs earth or air or fire or water, depending on the type of armor. In this case, slashing would go against the AL60; if you're hitting a nuker, say, with Pyromancer armor, the earth weapon would go against AL70, while a fiery weapon would go against AL80.
This is why melee and ranged fighters (and casters, too, to a certain degree) will carry a variety of weapons, quick-switching them to best suit their target.
As a general rule, it's elemental vs warriors and physical vs rangers/casters.
That said, there are many variables that can affect this; for example, there is W armor (forget the name atm) that gives AL90 vs elemental, as well as Sentinel armor which gives AL100 vs elemental. Assassin armor can be modded to give additional protection against various physical damage types. Rangers and necros have armor that gives + AL vs piercing. Necros have armor that gives base AL70, but takes more damage from Holy.
Melee and ranged weapons can have mods that give additional protection against various damage types- Defense (+ AL vs physical and elemental), Shelter (+ AL vs physical) or Warding (+ AL vs elemental).
http://gw.gamewikis.org/index.php/Main_Page is an excellent source of info for all things GW.
