The short answer is yes, you can paint over the rustoleum primer. The long answer is any topcoat is only as good as the base coat it covers. You could put million dollar paint on as a top coat but if the base coat fails it'll just be million dollar flakes of paint. Having said that, I've never heard anything bad about rustoleum primer failing when applied to a properly prepared surface but you need to be aware of the possibility.
I agree the primer is not the place to take short cuts
