I am a big supporter of playing FBS teams. It gives us more exposure than we'd normally ever get. It brings in a ton of money for the athletics department (I have to think some of our facility upgrades are as a result of the ones we've already played). It gives us a chance at a big upset.
I would agree that we should keep them to one a year though. Our focus should be on being the best NEC team we can be, trying to win the conference, and make a run in the playoffs. Playing too hard a non-conference schedule gets in the way of that.
I also think we should target the lower rungs of FBS. Playing TCU likely brings in more money, and the game is certainly higher visibility; but it puts us in a position of being sacrificial lambs. Buffalo, Hawaii, UMass, Coastal Carolina, Ohio, etc. we have a real chance of beating. We need to bring our A game and also hope they don't. If that happens we have a legitimate shot though. If we bring our A+ game and TCU shows up with their C- game they still probably beat us. If they show up prepared for us, they can practically name their score. I would make an exception to that rule for the occasional game with local teams like West Virginia, Penn State, or Pitt. Traveling halfway across the country to get blown out has to have a negative effect on morale though.