I think you are correct in your thinking...
1. This is a great game, and the more money BB makes, the more they will improve it, so that doesnt suck :-)
2. Private Leagues are a great way to play against friends or specific teams you want to compete against, as well as being able to try out/experiment with various tactics and strategies.
3. Its also a great way to experiment with various players playing different positions than you normally have them play to see how it works out.
4. They do not cause any gameshape, injuries for you like you thought...and they are played once a week on Fridays, and you can be in a Private league that has playoffs, a few teams, or a lot of teams.