There really aren't any 'fragile' classes in GW2, aside from armor types.(Heavy, light, and cloth) Considering every class can heal themselves, offer support, dps, and to some extents even tank. Every class can use ranged weapons/skills to the same efficiency as other classes. So a thief could go pistols and be as effective as a ranger with a bow. To put it in better perspective, a ranger can use melee weapons and be an effective melee DPS. Heck, I plan to run my warrior with a rifle/bow quite often, as even a warrior is very effective with range weapons.
Each weapon has it's own skill set, so if a class can use it, they can use it and be good with it.(Provided the player is as well) Thief would offer a number of options for defense and siege situations, much like every other class this game has at their disposal. If GW2 lives up to the claims, there will be no class at a real disadvantage from the others in any given situation.
I think the best example of this I saw was when ANet team took a party of nothing but rangers through an entire dungeon successfully.