I think I'd like this even if I were married, as long as the trial period isn't too long and interview experience indicates that I should have a good chance of landing the job.

Contrast with the alternative: I'm a senior, considering switching jobs, but I have no real way to evaluate the potential new employer beyond word-of-mouth and interview experience. What if it turns out to be a place I don't like and I just gave up my former position for something worse?

Working a few evenings & weekends (and getting a little extra $$) to evaluate the new company sounds like a superb way to gain the confidence and jump ship without regrets. In best case, I get a new job I like. In best case, I get some extra money and keep my old job which is better than the new one proved to be. Win/win? What's the worst that could happen?