For the sake of completeness, the two best counter-arguments I've heard so far (IRL):

1) MBAs are useful as a baseline business sanity tool, so you can get a decent employee to a point where they'll understand the basic vocabulary of a lot of different disciplines. For instance, they'll have a rough picture of what segmenting and targeting a market means, even if they won't know how to use it in practice, let alone compete with a junior marketer. Someone who's already read a bunch of stuff and managed a business isn't going to learn as much and might be disappointed by how close to common sense everything is.

2) MBAs teach you how to maneuver the minefield of the large company, where the decision-making process is complicated by personal alliances, office politics and employee/boss conflicts. To an entrepreneur, this will seem unreasonably complicated ("Why don't you just walk in a straight line?"), but someone having to deal with a fair share of Dilbert-esque behavior, additional ammunition, whether business lingo or complex models, might come in handy.

I should also note that I'm the only one with a largely critical view and that the rest of my tutor group is quite happy with the program.