There are two big gains with a minor league system. The first is the teams’ ability to have players taught the way they want them taught without taking up roster spots on the main team. You don’t worry if trying to season the player is dragging the team down because they’re on the farm team any winning the farm team does is just icing.
The second is it presents an opportunity to experiment with the game without risking annoying your main audience. The NHL does this all the time, most rules changes have been tried out for at least part of a season in the minors, and a lot of the things they try out there they don’t like the results and back them out. Sometimes even mid season which is something you just can’t get away with in the big money league.
I don’t know if these benefits make a need, but they could make a lot of things go smoother. Look at the recent debate in the NFL on extra points, all of the solutions presented are major changes that Goodell has basically admitted they’re afraid to adopt because they don’t want to commit a whole season to a new rule that might actually be worse. If they had a real minor league that’s a solvable problem. Also look at draft busts, a lot of the discussion around Tebow is people saying he should have gotten more chances, but his completion percentage shows there was a definite cost to giving him more chances, with a farm team he could have gotten a couple of seasons to be taught the NFL game without anybody worrying he was costing the team any games. There’s a constant discussion in the QB position on how different the college and pro games are and how few star college QBs have been given any of the training pro coaches want their QBs to have.
But somebody’s gotta pay for it. And I don’t see the NFL as ever deciding to foot the entire bill for a league they know nobody will watch. If they were willing to do that NFLE would still exist.