Hmm.. I think everyone has their own way to have sex.
Once you've been a couple for a long time, you get into a routine and know what works for each other. So, one can be a pillow princess 90% of the time and top the rest of the time, when the mood is right, and it's perfectly ok. It changes the routine.
If you switch too regularly, it loses its novelty value and it becomes just another routine.
But I agree that big differences in libido spelled the end of more than a couple.