Well... a specialist makes it easier for the team to engage in something else while still getting that important XP. But ive met specialists that think they are something else..... they can lose the game just by them selfs. But a decent specialist can double the xp income and outlevel the other team easily.
BUT, that being said. I truely hate stealth characters. I really hate them. I have yet to meet a good one that can do something really usefull. They just tend to die all the fucking time. But I havent hit 40 yet either, so i cant judge them before a serious high ranked match....