1. It is a regulatory problem if it is not the pilot taxiing the plane.
2. There is a driver needed to bring the tug back.
It is about saving money on big and busy airports (imagine ORD) . You save maybe 30-45min engine runtime (fuel and engine time). Of course the tug needs a lot less fuel than 2-4 big jet engines running in ground idle. Of course that is good for the environment as well.