I use my test stand. Mostly for engine break in or after a 1 point spinner landing. Doah!

No mess to clean off of the plane.
Completes access to the engine.
Smaller than the whole plane to cart around.

Just a handy tool for engine work.

No all the setup isn't the same as in the plane, but you get everything close so when you do run it in the plane you can fly and not play with it in the pits forever. Or worse, sit on the flight line while other folks are waiting to fly cause your engine just dosen't sound right, to you!

Just my opinion.

Al