Yeah, I mean I love GT5 and Forza and actually miss their graphically slick menus, music, and my large friends list. I actually think they look good too. But the triple screen setup is a pain, the hardware limitation is a pain, and like Latte said once you add motion and proper transducer effects they really are inferior.
At trade shows gt3 and gt4 had full motion (haven't seen gt5 with motion) and Forza still has motion. How much effort would it take to make that part of the full game.
It may not be a real money maker feature but triple screens probably isn't either. If Sega f355 challenge had motion you could guarantee Forza would have had motion because that is essentially the sole reason for Forza's triple screen support.