I think the key problem many people here appear to have with multiclassing is that they see character concepts in terms of single classes.
Classes are a metagame concept to begin with. You use them as a means to an end. If you want/need 3-4 base classes and 2-3 prcs to flesh out your character's capabilities, I don't see why you should have to settle for anything less. Conversely, what if I have a great character concept, yet am forced to stretch a single class to try and accomodate it simply because my DM would allow multiclassing? In the end, he has few/none of the abilities he ought to have, or cannot live up to his own fluff. This seems contrary to the spirit of "roleplaying".
I also think that too much emphasis is being placed on the "prestige" aspect of prestige classes, but that is another discussion for another day.
For example, let's say I wish to play a monk-like PC, inspired by some wuxia or anime show. But the PHB monk stinks to high heavens, and does not really possess the abilities I have in mind for my character. After some deliberation, I come up with a warblade1/monk2/warblade+5/swordsage1/warblade+2/swordsage+1/warblade+3/masterofnine5 build, which is not only fun to play, but can also hold his own, and matches my vision of what he ought to be capable of achieving fairly well.
Conversely, I see no reason why I need to roleplay my own 3classes+1prc monkish PC any differently than if I had been playing a monk class instead. Both would still be monks in the story sense, in that they live ascetic lives, train in monasteries and adhere to the same code of conduct. Any backstory you give your monk, I believe it can fit my monkish PC just as well. If anything, I daresay my creation is more monkish than the monk in virtually every aspect!
Bottom line: Multiclassing does not always produce overpowered PCs, nor do I need to multiclass to break the game. In the very least, I have yet to see a melee build that can trump a single classed wizard, however aggressively you multiclass him. It just helps to narrow the disparity. A little.
Discuss.
Classes are a metagame concept to begin with. You use them as a means to an end. If you want/need 3-4 base classes and 2-3 prcs to flesh out your character's capabilities, I don't see why you should have to settle for anything less. Conversely, what if I have a great character concept, yet am forced to stretch a single class to try and accomodate it simply because my DM would allow multiclassing? In the end, he has few/none of the abilities he ought to have, or cannot live up to his own fluff. This seems contrary to the spirit of "roleplaying".
I also think that too much emphasis is being placed on the "prestige" aspect of prestige classes, but that is another discussion for another day.
For example, let's say I wish to play a monk-like PC, inspired by some wuxia or anime show. But the PHB monk stinks to high heavens, and does not really possess the abilities I have in mind for my character. After some deliberation, I come up with a warblade1/monk2/warblade+5/swordsage1/warblade+2/swordsage+1/warblade+3/masterofnine5 build, which is not only fun to play, but can also hold his own, and matches my vision of what he ought to be capable of achieving fairly well.
Conversely, I see no reason why I need to roleplay my own 3classes+1prc monkish PC any differently than if I had been playing a monk class instead. Both would still be monks in the story sense, in that they live ascetic lives, train in monasteries and adhere to the same code of conduct. Any backstory you give your monk, I believe it can fit my monkish PC just as well. If anything, I daresay my creation is more monkish than the monk in virtually every aspect!

Bottom line: Multiclassing does not always produce overpowered PCs, nor do I need to multiclass to break the game. In the very least, I have yet to see a melee build that can trump a single classed wizard, however aggressively you multiclass him. It just helps to narrow the disparity. A little.
Discuss.