I think professionals who work doing weddings, events etc. will have primes because the quality, as everyone states is superior.
For the other 99%, its all about convience...and a prime lense is NOT convenient. It is human nature, that if you have to go a few extra steps, then you just wont do it.
Personally, not having a zoom would be a complete Pain in the rear. If I was a pro doing portraits, then i certainly would have some primes...but guess what....99% of us are not pro and just take pictures as a hobby.
I'll be getting the 50mm 1.8 soon. This will be the only prime that I ever own unless I find one laying on the ground or get one as a gift. The only reason i'm getting this one is because my wife owns a daycare center (160 kids) and on holidays the children get their pictures taken with santa, easter bunny etc.
The question is this, are you trying to make money off your pictures? If the answer is no, then the IQ between the prime and a good zoom are not going to matter.
This is all just my opinion and not based on anything I researched.