Evidence for base+patches being the preferred form:
1. RedHat used this form for maintaining (in the sense of humans doing the work) the source of their kernel RPMs
2. RedHat *still* have, at least, processes using this form, if not humans
3. Other corporates *also* have preferred this form (e.g. Sun)
It'd be interesting to know whether or not RedHat still internally are using the base+patches src RPM form for doing maintenance work. Your comment seems to suggest this is a possibility. In which case, RedHat really ought to be releasing the base+patches. It's really hard to argue the base+patches are NOT the preferred form if that's what you're using internally to maintain & build the distributed binary RPMs..