I'll list a few of the pros of Luigi in doubles
Pros:
Amazing kill power. Fsmash, Up'B, Nair. Misfire even. They're also much easier to hit in doubles as the opponents aren't using 100% of their attention to avoid them. I remember some games I played I just randomly ran away from the guy I was fighting and up'b the guy my partner was fighting.
Can stock tank pretty well with his good recovery, its predictable, but thats what the partner is for
Has a projectiles
If your partner gets a grab near you, you can net an easy kill at 40-50% with UpB.
His Nair hits you straight up, so if you're in trouble he can dive off stage and just nair you.
If Pit can get Luigi inside, he'll wreck em. Luigi doesn't really have a hard time killing, or racking up damage, but he does have trouble getting into range.
Cons:
Luigi is floaty as hell, he'll probably get in your way if you're trying to camp.
His projectile kinda sucks, so he won't be able to camp effectively with you.
Has a BAD MU against MK, and once hes off the stage you're going to need to save him or MK's going to take that stock so easy.