In BG1 absolutely no: except for solo'ing, you don't gain enough experience to make it worth it.
In BG2 yes, it can be, but makes much more sense with a smaller party: a large party has every angle covered, and multiclassing is unnecessary.
That depends. Just because someone is multiclassing doesn't mean they're trying to cover multiple roles. For instance, a fighter/mage or cleric/mage is the best tank in the game, much better than either a pure fighter (self-only buffs) or a pure mage (a lot more hp and ability to have a good AC).
Also every character who's not a mage needs to be decent at melee combat, and having fighter levels (or ranger for a cleric) really helps with that.
A fighter/thief is a much better character than a pure thief in BG 2 since you don't need a lot of thief skills and pure thieves suck at fighting. A ranger/cleric can also be better than a pure cleric, both for fighting and for spells (since you get druid spells too), but dual classing is probably better for that.
To the OP: Frankly, once you understand the rules of the game, and know how to get past the difficult early levels, BG 1&2 are easy enough that you can play whatever you want and still win, so just pick what seems the most fun for you without worrying about other things.