reezer
macrumors member
Probably because 8GB DIMMs are only available as registered ECC. Apple decided to use only unbuffered DIMMs, so they would have to change to RDIMMs as they can't be mixed with UDIMMs. Well that or offer both and trust the consumer to understand.
Now that I think about it, the Dell I use has 8 DIMMS that are expanded to 16 DIMMS via two riser cards everything ECC. I would suspect that the MacPro is too crammed to for that setup.