Of course I use a zoom. A few in fact. I use a 80-200 F2.8 L and a 18-55 EF-S.
What I meant is that I won't use a zoom to replace my 400mm F5.6 L as the other poster asked. Only the 400mm F5.6 L give me sharp enough shot wide open with a teleconverter.
I use a teleconverter when the subject is way too far but I prefer not too. The teleconverter works better on a prime because the TC show much more the image degradation of the zoom and since the zoom is at its best stopped down, that farther reduce the amount of light. With the prime, you can use it wide open with a TC and you still get sharp images.
I won't use a zoom. I need a lens that is tack sharp wide open and only a prime give me that. I still have my 400mm F5.6 L and my next move will be a 300mm F2.8 L.