We have found that some people like using the service as an optimization/development tool. Starting a load test is painless enough that they can write some code, test it with our service, change some things, test again, etc. and sometimes get huge performance gains. Buying another server gives you a 2x performance increase, but it will cost you forever. Optimizing your code can give you 20x performance increase and only cost you while you're working on the optimizations.

But I guess it depends on how you use the service. For people who just want to verify performance occasionally, what you write makes sense, and we're going to create a one-time (or maybe "one-day") offer also that you can buy every time you need to run a test.