Your A-spec and I think B-spec points "unlock" the arcade mode tracks. The further you progress in GT Mode and the more circuits you open up in GT mode (in the Real Circuit, City, Original Courses,Dirt & Snow) those same open up in Arcade mode.
You also get A-spec or B-spec points in arcade mode that apply to GT Mode.
I'm not sure about the cars. I haven't gone into Arcade mode much, rarely did in GT3 too, but have noticed a few more cars available now that I am around 30% since the first time I ventured there with a 0% completion. I think whoever said that why you buy or win a car in GT Mode it opens in Arcade mode...that seems about right.