I think the gist is contained in rc pete's last sentence. Some of the non-pros are not very good at letting a faster car by, (and I'm among the worst), and some of the pros are so competitive they tend not to see the slower cars at all!
In essence, the separation provides a basic pre-sort, and racers are free to move up when they feel ready.
If the pros and nons were to be raced in one class, it would still be better to do a pre-sort for the overall enjoyment of both.