I think the use of FB-DIMMS is most of the problem. It doesn't seem that iMac users have near the number of issues with ID as MP users.
ECC is going to cure problems, not cause them. Won't fix bad code or corrupt files, but will prevent those from happening.
See today @ www.macintouch.com on help for problems.
Also, not everyone uses small files, and for those dealing with 1-5GB images (aerial photography or whatever) even a Quad G5 breaks out in a sweat. Not your average by a far shot, but the type of person looking at moving to Mac Pro.