As sum one mentioned before, mesmers dont have the appeal of other classes. Tanks can go whith crazy weapons and armors and sustain TONS of damage, rangers can tame a pet and trow arrows and so on, monk can heal and protect and keep the team togheter, elemental can take down TONS of dmg at once in a large area whith glory effects, and so on... Mesmer for instance, oo well... they can do that pretty purple aura thing ^^
I would say also the armor make em a poor PVE choice, and since they are not MASS DMG dealers or MASS MOB CONTROL their job into PvE is just ripped off to fast casting his secondary class skills and stuff.
Another thing against the mesmer is the dificult to control one, and truth be said, i would ONLY recomend you building one after u already got enough playing whith other classes since u actualy gotta no what patterns of build your enemys are running in order to counter them properly
As necros once were, mesmer are NOW the least played class on PVE and PVP, simple because these are "non regular" classes, necros cant trow away TONS of dmg in one target, nor mesmers can take agroo of 40 - 50 monsters and hope to stay alive, but still, knowing how to mannage both classes makes em worthy adversaries... The necros had surpass this barrier, even thought i still see a lot of HORRIBLE necros running coockie cuuter builds and thinking they are invincibles, and truthly i think the mesmers will get the same chance, maybe better armors would help but whathever ^^
Later.