having more of the "good" stuff go free more frequently
Sure, everyone likes a free lunch, but doing that would dramatically remove the incentive to subscribe.
There are a few gems in there, but any real gamer already owns them
The real benefit of the service is that there are plenty of games to play for one low fee that one would otherwise never pay full price for. Yes, for the real gems with replay value, it is more economical to buy them outright. Where Gametap makes sense is for all of those games that you would like to try but don't want to spend the money on them. If you are not using piracy for those games, then Gametap is a great deal.
but the current system cannot hold my attention on a monthly basis.
It really depends on how many games you typically buy and play. If you are someone who tends to buy a lot of games, play through them, and then never touch them again, Gametap is a great deal because you'll spend a lot less for the same play.