We still aren't in a position to schedule 9-10 P5 teams a year, so it's a very smart move to schedule the "best of the rest." UCF, Houston, Boise State, Utah State, NIU, Fresno State, etc.
SOS will reign supreme with the playoff format, even with any potentially undefeated season. There's always a handful of G5 teams that would smack around the bottom 3rd of the P5 teams, and it's very smart for TH to schedule these series.