Here's a good reason: I have a real old (68?) S.T. .40, that I thought was in good shape. I could have mounted it in a plane after the couple of tanks I ran through it when I got it. (A couple of years back)

I just re-mounted it on the test stand "just to make sure it was gonna run O.K." and guess what? I apperently didn't do as good a job at storing it as I thought for the poor storage conditions it was in, and the bearings crapped out (technical term) while I was running it today...at least it didn't cost me anything, and it didn't quit in the air. (not that that is the death knell for a plane, but it could be for someone who is a beginner.)