Just because its rare (especially in college), doesn't mean it can't work. John Harbaugh only had STC roles before becoming a head coach. The Chief's STC Dave Toub was interviewed by multiple teams for HC spots and has only been a ST guy.
Guys like Marv Levy, Dick Vermeil, Bill Cohwer, & Bill Belichick did have short OC/DC roles, but came up mainly as ST guys.