It would not. The only thing I could imagine countering is: Fly me to Austin for two weeks, put me up in a hotel, and hire me to do a project at $x/hr. If we both like what we see, let's talk about a full-time position.

Frankly, I think this is anti-candidate behavior if the employer suggests it, but if you actually want it, they should be willing to do it if they can vet you otherwise (github, etc).