1- as others have said, I actually use sprint in combat so it might be on cd (and I'd hate it being on cd during the fight because I used it on trash)
2- more often than not, tank needs some time being forward because stop to aggro a group, go again, stop and grab another and so on. As a healer or dps, if you go as fast as the tank you're often standing there waiting, since you shouldn't heal or dps until aggro is established.
It's all very situational. Of course as a healer I try to be gluey, but that might lead to the "too early" consequence.