The thing is, the carburetor/tank relationship effects tuning. If you run an engine on a bench, you've already established one engine/tank relationship, and you've tuned accordingly. When you transfer the engine to the plane, you can try your best to get the tank on the same level. Invariably, the setup will differ in some subtle way, and you'll have to retune all over again. You're duplicating your efforts. Why not establish the correct carburetor/tank relationship once, in the plane, and then leave the mixture alone forever? Its one less variable to contend with.