It's interesting that my Canon EF 24-70mm f/2.8L USM can only use a single double-cross AF point (group B lens), and that my Canon EF 100mm f/2.8L Macro IS USM can't use *any* double-cross AF points (group C lens). Both of these are fairly recent EOS lenses, and I was led to believe that the five centre double-cross AF points were usable with f/2.8 or wider maximum-aperture lenses.
For the case of the macro - it must be due to the really close focusing distances. Still, it should enable group A functionality from 0.5-infinity, and group C from 0.3-0.5, but I guess it defaults to the lowest common denominator. Since I don't shoot sports or track subjects with this lens, it's a moot point really.
For the case of the 24-70, I wouldn't be surprised if the reverse zoom mechanism has something to do with making it a group B lens and not a group A lens. Unfortunately the 5DIII manual doesn't mention what group the new Canon EF 24-70mm f/2.8L II USM lens is in, although I'd be very surprised if it wasn't a group A lens. Since I'll be upgrading to the version II lens, I can live with this. I do use this lens for some sports work (on the second body) so it's important to me.