Why? Well, Darcy over at Art of the Drink has some pretty good reasons and even got in to some back-and-forth with a PR Moron at National Bartending Schools. Morgenthaler has some good reasons too (and a way to get a bartending job without that experience). Joe Bartender went to a school and wrote about his experience (including a conversation with an instructor at a school who though the curriculum was crappy too).
I did attend National Bartending School and did get a very cool little bag with a cocktail shaker, a recipe book, a jigger, and a strainer in it for my troubles. That's pretty much the only thing I came away with of any real use.
Do you doubt? Go talk to the head bartender at your favorite bar (No, not at TGI Fridays) and see how they got in the business. You'll be hard pressed to find someone who did it via bartending school and if they did, they'll be embarrassed to admit it because they know it was a waste of $500.
Normally, I wouldn't pipe in on something like this, but National Bartending School's response to Darcy was just ridiculous.