hi,
of course you can use a lumped port. But the problem is that there is no lumped port in reality. The field looks different that it looks at your "hardware" antenna. Therefore you simulate a discontinuity because the field has to change from the lumped port to the microstrip. That can be very important if you want to compare sim and measurement. I found out that a waveguide port is always clother to the reality and that is what counts for me.
Everybody may have diffrent tasks and applications. Therefor an lumped port is at least easier to create and gives still good results.