Almost all of the time, zooms win with me for the advantages of less lens swapping and/or less of the cropping needed when the best available prime is significanlty shorter than ideal for the composition.
Convenience is a major factor in my purchases, too. But I almost swapped my 50mm f/1.4 (and some money inbetween) today for a 24-70mm f/2.8L, until I suddenly recalled a few shots that would be really icky to take without f/2 or better. I'm still getting that zoom, though, since I'm an equipment freak.
One other case for primes is at extremes of telephoto such as for wildlife photography. The tendency then is that, prime or zoom, you are going to take a lot of shots at the maximum available focal length wth no flexibility to get perfect framing, so that ther wil often be significant cropping. Then what counts is having the longest, sharpest lens. For that a prime is likely the best choice, and can also save some weight.
Is it only me, or are there others who think there's a hole in the prime lens lineup around 200mm? Sure, there's that chunk of L glass, but there's no IS! I'd really like a 200mm prime with IS. But maybe everybody who wants that just buys the 300mm f/4L IS instead.