Back in October I wrote about the reasons why I've given up zoom lenses (most of the time) in favor of primes.
To continue the discussion, I just ran across a recent article by Mason Resnick posted on the Adorama website. Mason comes to the same conclusions I've come to, using objective measurements and evaluations. He compares a 50mm prime to two zooms shooting at 50mm. One of the zooms is a kit lens (cheapo) and the other is a pro-quality zoom (expensive).
In overall performance, you get what you pay for. Only the expensive pro-quality zoom comes close to or equals the prime. But there's one exception: light transmission. He doesn't really go into this in the article, but the data he posts show that the prime still wins in that category. So, for low light photography when you need the fastest lens possible, prime is the only choice.
Since you can buy two really decent primes for the price of one high end pro-quality zoom, why not get two primes at the extreme ends of the zoom's range, and use your feet to adjust for the intermediate focal lengths?