Phil Snow had been coordinating defenses for over 30 years when Rhule hired him at Temple. The dude has been calling defenses since Reagan's first term. From the time Rhule met Snow at UCLA, Snow coordinated at UDub, LBs for the Lions, and coordinated at EMU. Was with Rhule at Temple, Baylor and Carolina, where he fielded the second stingiest defense in terms of yds/game.
A dude that had already done most of the coaching he was going to do joins up with a young coach who is going somewhere and continues to perform in a consistently excellent way = "comfort hire". In a sense you are right. Rhule, Butler and the rest of the defensive staff are very comfortable having him out on the practice field, where from what I've heard, he is doing a lof of work with the young DBs. But then again, he really is just Matt's buddy, right?