There are some items to unlock in Sim Mode that make Arcade Mode more fun. If you win the Superlicence, you unlock all the tracks. When you win races in Arcade Mode, you unlock certain cars for Arcade use only (many rally cars and the S-class cars). But there is no Hi-Fi Mode for GT2.
And the AI isn't as much fun to beat in GT2's Arcade Mode, but you can set the number of laps. So there is some value in winning all the tracks, this way you can do time trials.
Hope that clears things up.