The past page all seems to revolve around treating classes as careers or professions rather than abstract game artifacts. My group(s) tend towards the latter: if you want to play a knight, you're welcome to take the knight class and be done w/ it, and make it into part of the narrative, but you're under no obligation to do so. We treat them as bundles of abilities that are linked together in what is one of the charming, and occasionally simplifying, parts of the D&D game.
If you do that, stuff like the RP exchange above -- which we've all had to suffer through and would annoy the hell out of me -- doesn't really even make sense. It's hard to talk about what a "sorcerer" would or won't do.
I gather from the discussion so far that I might be an outlier in treating classes this way, but I'm going to argue that it really makes for better games, namely by making for a better character creation experience. First, it's not compulsory: like I said, if you want to create a "barbarian" and just grab levels in the barbarian class.
But, let's take a fantasy series I happen to think is quite good and w/ well-realized characters, George RR Martin's series (at least until "Feast of Crows," but that's another conversation). You wouldn't be crazy if, by profession, you were to say that Sandor Clegane, Gregor Clegane, Jaime Lannister, and Rob Stark were all knights. And, if class = profession, then they are all going to be knights/fighters or something thereabouts. Now, maybe feats will allow you to get some difference at the game table between the Mountain that Rides feels in combat and how Jaime "I'm better w/ a sword than you and your 10 best friends put together" Lannister, but maybe there's a random class ability here or there. It certainly wouldn't be off to characterize the inhumanly strong, huge, and berserk Gregor as a Frenzied Berserker or some other flavor of Barb even though he's lived in the bosom of civilization the entire time.
That, I submit, is what dipping gets you. Sometimes feats and magic items will get you to the same place, since powers are essentially randomly distributed throughout the game, but it seems weird to treat classes as the one sacred cow. Also, feats might just not do enough to make the characters feel distinct. Gregor is big, so Jotunbrud is a good choice, but if that was the only difference between him and another character you wouldn't be selling how the character is described at all -- that would be too minor a difference.
@the "More Powah" point
I think others have articulated my annoyance w/ this point better than I have. I just want to add one more thing. Just b/c you can in theory build the concept isn't necessarily enough. If the non-dipped version of the character sucks, meaning it can't really contribute in a reasonable party, then it's not really viable.