I think it is a good idea to allow makes that don't fit the traditional GT mold. More people can identify with these cars, which means there is a potential for attracting more fans.
I think Grand-Am had a good idea with the silhouette cars; however the restricting nature of the rules limited the potential IMHO. The RX-8s, GXP.Rs, and other tube cars are a tad dull compared to some of the daily driver mutations seen in the past-or currently in Super GT.
Having Mustangs, Camaros, Hondas, whatever competing at GT2 speeds could make for some radical cars, injecting more cool factor into the sport, not to mention more variety.
Hopefully Grand-Am overhauls its rules and teams like Turner will be racing in GTs that look more like this:
http://pictures.topspeed.com/IMG/cro...3-1_460x0w.jpg