Mechanics is the skill that u use to pick locks/disarm traps etc. so it can be done by other clases than rogue. The main advantage of Rogue is that he is best one target DPS class in the game.
There is no rogue, barbarian or monk companion in game, so it's good to chose one of them as main char ( i chose rogue, because i never play in BG/IWD as one).
I hope they add 3 remain class companions in expansion.
Idle companions can get exp when they do a stronghold missions for you, normal quest exp is only for main team.