I can see where you're coming from. However, as far as .NET is concerned, all of the class names are semantically relevant. It's not like the class names are "blackBorder" or "redBackgroundOfAwesomeness". The class names are things like AspNet-Details-View, which is a completely semantically correct class name.
Now, if, for example, you have your DetailsView render in a dl/dt/dd structure (like we've changed our adapter to do) but have that dl render the same way as every other dl on the site, then yes, the class names become superfluous.
Also, keep in mind that the adapters are meant to replace rendering for default controls directly with no code changes (other than an external CSS file, perhaps). If classes aren't included by default, that breaks. You have to go to every single one of your controls and add a class. For sites that have a lot of applications, that's a lot of time and a strike against the adapters. (Some things would be impossible without the class hooks. If IE's CSS selectors were up-to-snuff, it wouldn't be as big of a deal and it might be possible to get rid of all but the top-level class names.)
Are the default classes really not WACG compliant? They should be semantically descriptive; although they do relate more to .NET concepts than web page concepts in general, it's still safe to think of a set of HTML elements as a "FormView" and make class names to match that.