I think Tourist Trophy was a way for PD to learn to model human actions and interior views rather than an actual project to do a motorcycle game. The hook of bikes got enough of us signed up as beta testers for for what became GT5 features. By doing it as a separate game they avoided the problem of too many polygons to do it in GT4, which would require smaller fields in races, which would have been seen as a step backwards in the game.
The bike actions were so unrealistic (leaning without countersteering and no way to crash the bike without actually hitting something) as to be irrelevant, but with the riders out there in full view they did have to learn how to model them. TT also had the first "in-car" view, if you want to call it that, where you had a realistic rider's POV, with windshield, working gauges, and so on. This was never in GT4, but appeared in GT5 on premium cars. Even so, the polygon limit on the PS2's graphics made the actual racing rather boring, 4 bikes max, and incredibly stupid AI.
That said, I enjoyed the game, even though it only took a few sessions to complete it and run out of stuff to do.
