ACtually, it looks like they have done this very thing with the iPhone game, and it's pretty sweet. First of all, you have to buy the game so it's never been free, even though lots of us bought it when it was only $1. Now they've added an in-game store where you can add a new brute to your dojo for $1, or 3 brutes for $2. You can also buy 100 fights for all of the brutes in your dojo for $1.
So for me, I had already unlocked 7 brutes and bought 3 more and now I have 10 in my dojo. I can buy a booster pack that will give 10 brutes 100 fights, meaning 1000 fights for $1 in addition to the 5 I get per brute per day. This would effectively give all my brutes 20 days of advancement and would probably take me all weekend to play out. A MyBrute marathon weekend for $1? I think that's not bad. What if you could pay $1 and just keep playing your web game brute and all its pupils non-stop for a weekend?
Well, I realize that with the super high level brutes, there's not much competition in the arena to keep you busy. But bear in mind that there's no bots in the iPhone game, no cheating, no IP tracking, and the top-level brute in the game is only 188.