the main reason I am building a new test stand is most of the planes I have now are cowled, and I happen to have several of the same type of engine that I have bought used,or rebuilt.
before I mount one in a plane, I like to go over the engine, check the valve lash so its in specs, adjust if necessary, and then test run it, check for leaks. and any problems like exessive oil spitting out where it shouldn't, and that its running at pretty much perfect tune, then I will mount it in my 1,000$ airplane.

its a matter of economics, if a engine isn't running spot on, I will go with another one and figure out whats wrong with the rough runner, getting oil all over my airframe is a bummer to clean up as well, deadsticks can be really costly.

I can accept flying a plane into the ground under power but gliding a powered plane in and crashing because i didn't make sure the engine wasn't running perfect inexcusable,and some engines take a little time to get just right so I like to stand option.