For the Xbox, I thought PGR2 might have been a must-have game, but I was ultimately disappointed by lack of wheel support - or rather, wheel support on GT4 was so outstanding that it crushed all benefits of the Xbox experience, never mind the accurate Nurburgring, 60fps and other stuff.
PGR2 came out a year and a half before GT4 here in Europe... Your argument sounds kinda unreasonable to me. Plus Forza Motorsport and Rallisport Challenge 2 were both fantastic games. Xbox was definitely the last-gen console for racing gamers even despite lack of good wheel.
I think the 360's killer app is right now Xbox Live (also with silver account). It enhances gaming experience so much that playing on older consoles feels boring sometimes without messages, demo downloads etc. It has a solid library of games, but the best have definitely yet to come.