If you have a spare frame it would certainly work fine as the support structure. I have some extra frames but chose not to for due to the added bulk and limited access between it and the body to work around and access for sand blasting, scraping, any potential cutting and welding, DA sanders, and painting. Having 6-8" between the support frame and the body helps a lot with this IMO.
However, if you only have the one frame and use it for the support, it confines the flexibility you have on your project. It is nice to be able to work on the rolling chassis and body as separate items so when one is stalled waiting on parts or some other work, you can work on the other.
We always knock out the rolling chassis first and have the engine, trans, suspension, fuel and brake lines, wiring, etc. all done and ready for the body to be dropped on when the body comes off the rotisserie after paint.
The frame we made probably only has $50 in material and maybe 4-5 hours to make it. But like I said, it is made so I can easily drill new holes and add new cross members with posts for any other vehicle, so it also adds greater flexibility than needing a spare frame for each vehicle we do use on the rotisserie. It also breaks down for when it is not in use so it is easily stored on my steel rack or in the rafters.
We pretty much only do Broncos, but we be doing a Corvette for a guy after the current Bronco is off of it.
Like was mentioned, when you add up the materials and time, to build one with the same features and flexibility, you are not that much ahead to build one from scratch than to buy some of the ready-built models. I think this one was about $1100 (bare steel, we painted it) at the time. There are import knock-offs that look identical that were $850 or so though places like Greg Smith Equipment, but it looks like those are now $995. The Auto Twirler Plus model is now $1299 in bare steel. They also make a basic model for under $1000.
http://www.gregsmithequipment.com/Atlas-Auto-SPINS-SIMPLE-POSITION-IN-SECONDS-Rotisserie
http://www.autotwirler.us/product-p/plus.htm
Whichever way you go, any of them beat the hell out of working on your back on the floor with crap falling in your face! ;D