I can see where overhead becomes an issue though too
I can see manualy managed mac filtering being an issue in a business with many wifi workstations. Inet Cafe's would not be a good place for mac filters either. Universities enter the mac when you register for there network usually so that works too.
Business clients would need a slick, shinny solution though so your looking at something that will "scale" (as they say).
Guess it really comes down to; does the client want security or does the client want to play at security. Either way, it's there network, we can only inform and ablidge.