My thinking, usually, is that I'd rather spend money on optics than mechanics. Modern zoom lenses can be really, really good, but all other things being equal they tend to be more expensive than comparably good primes.

That said, I've got a Kiron zoom lens in FD mount that I just love. (On the other hand, I believe it cost me US$8.99, so it's a pretty clear exception to the rule above!)