I must say, I disagree with all you folks
For instance, try the monk: at high level is a butt-kicking machine. Lots of attacks, packing a heavy punch (esp. if you max STR out with the girdle of giant strength or the gauntlets of ogre power - I have my monk w/the girdle and Keldy w/gauntlets and Carsomyr), Stunning attack (a big plus), Quivering palm (the 2 o 3 times I've seen it WORK actually - but it's the old story about hi-level saving throws
Single class characters, esp. if they're spellcasters, have more spells (and this comes in handy for summoning matters - see Edwin vs. Aerie or Viconia vs Jaheira).
Now, it's good having a sort of "backup" power for tough battles, but I still strongly believe that a bunch of single-class, very focused chars acting together can bring down everyone nice an'clean
At the present, my party consists of:
-main guy, human monk 19th
-Jaheira, 15th/15th
-Viconia, 19th
-Edwin, 16th (Guy, he's evil, bu he ROCKS!
-Keldorn, 17th (main guy's all-out brawl buddy
-Imoen, 16th (backup mage - too cute to kick her out, and bad roleplaying also - my main is lawful good)
So, what do you think, guys and gals? Let me know...I'd gladly start a thread about this