I never started GT4's Arcade mode until last summer, and that's only because a bunch of us were doing online Continental Tire Championship mockup races. Arcade mode lets you set the number of laps, and race against other Ai cars. We'd do a race by Sunday, PM our times to Super Shouden, and he'd post the winner & runner ups the next day.
...but I never really got into GT4's Arcade mode other than that. I don't know if they're "taboo" or not. I just never saw the point of doing Arcade. I did enjoy GT2's Arcade mode, though, quite a lot. I think I completed GT2/Arcade, but not any other GT.