It's a realism vs. mechanics issue. Realistically even sniper agents should be trained in some stealth type abilities. However in terms of game mechanics stealth is a gap closer ability used by melee DPS to close in with enemies and not face fire while getting in range. As snipers endeavor to engage from max range it is from the perspective of game mechanics pointless to have stealth.
In PvE we engage outside of aggro range. Stealth has no use. In PvP as has been explained it's limited utility would be exploitative and unbalanced really only useful for harassing people (mostly lower level players) in open world PvP situations.
The only real benefit of stealth would be to stealth past mobs in some quests however due to mob spacing and our range of engagement it would mostly lead to more chance of pulling adds since we need to engage from range not in the same space as a group of mobs.
I'm with you on the PVP thing but, couldn't it be argued that while a Mara uses that ability to close the gap between an enemy for melee, a Sniper could use that ability to widen the gap between him and another enemy buying the sniper precious time to continue his long range attack or channel an ability or to just plain escape from a sticky situation.