There is a difference between unlocking a skill and learning it.
Unlocking: the skill is available to use for all Heroes and PvP characters. You can unlock a skill by using Balthazar faction or acquiring the skill on a PvE character.
Learning: PvE characters need to learn skills. You can do this the regular way by completing quests that award skills as a reward or buying them from a skill trainer. However, PvE characters can also learn unlocked skills, either by buying them from a skill trainer from the same campaign the skill is from (i.e. an unlocked Factions skill from any Factions skill trainer, a core skill from any skill trainer etc.) or by using a skill tome. Unlocked elite skills can be learned with an elite skill tome (or the regular way by capturing it from the appropriate boss).
As for point 2: unlocking heroes in PvP is completely unnecessary. Heroes can't be used any more in PvP and PvE characters need to acquire heroes by themselves.
In short, unlocking a skill has as a direct benefit to PvE character only that their heroes can use it. As an indirect benefit you have the opportunity to buy the skill from any appropriate campaign skill trainer or learn it with a tome. Unlocking heroes is redundant and has no benefits at all.
P.S.
Wiki information on unlocking.
P.P.S. Unlocking items (runes, weapon mods) is only beneficial for PvP characters.
too bad my pvp points were spended in trash...
