A Tank not sprinting often means your stacks, buffs or procs fall of as a DPS, which is quite infuriating to deal with and slows down the run significantly. Just sprint. I always do when I tank, and others should as well.
I once had an amazing DF group in an expert roulette in terms of DPS and Healer damage, but the run overall still took the average 18 minutes instead of the fast 13~15 minutes, because the tank was dilly dalling around, never sprinting.
