I think multiclassing and spellcasting IS meant to be crap, and I support it. When you are a divine spellcaster, you have to focus on your deity, and thus sacrifice some fighting experience warriors gain on the battlefield. And I must say that both the 'pure' Cleric and Druid are not 'weak', and do really not need to multiclass because they can also fight quite well.
When dealing with arcane magic, a fighter/wizard just does not fit well in my opinion. When studying the arcane you have to spend years and years of research in hidden dusty tomes located in isolated and dusty places. After your study you are the most powerful spellcaster of the party, and at high levels probably the most powerful character. You just can't fight well, but that is what wizards are about.
When considering the sorcerer, yes, he doesn't have to study, but he also relies mainly on magic, and thus has to sacrifice fighting skills, for the sake of game balance
Last but not least, the Bard is just fine: Jack of all trades, master of none.