my car is a full tube frame. I stuck the subframe under the stock unibody, and it seemed to me that the front a-arm mount would try to occupy the same space as the "frame rail" where it curves up behind the tool storage. As for the reasons for doing it, mine have mostly already been mentioned, but modern suspension, easy LSD, easy disks, easy 5 lug conversion, lots of parts available. Of course easy is relative, but when you are doing a tube and all you have to work out are the subframe mounts..... I will try to get some pics and post when I have them.
Jason