I think Karting should stay.
In GT5 it only had it's own little Special Event. This didn't express enough how important Karting is in motorsport and that it's the stepping stones for most drivers. It would obviously need improved, as with a lot of things. If there was such a thing as it's own series, that would be a big step forward.
Karting has so many different skill levels and leagues that could be implemented into the game to make a worthwhile series to compete in. Currently, you can only race in Beginner, Intermediate and Advanced - but there isn't any difference in them anyway, just the prize money.
The progression should be made more difficult and the challenges that follow. Karting could be a great thing if it's managed right. So, I think it should stay. Plus it gives us something else to do.