I'm not in favor of it to be honest.
If they allow it, it should be heavily restricted since their classes are part of the companion's personality. I can imagine multiclassing as an option for companions though, e.g. Minsc (once we get him) as a ranger/barbarian since he is meant to be a Rashemi berserker if I'm not mistaken.
Yeah, he's pretty much a barbarian berserker, except he was introduced as a character before the class was an actual thing available in D&D (or at very least in Bioware's D&D, can't be bothered to check at what point 2nd Edition introduced the barbarian in pen & paper).
Still, I'm guessing that at this point he's pretty well established as a ranger in everyone's imaginary and they won't change that.
