This only works in specific situations. If you're all level 50 running totorak on duty roulette, what the hell is the point of taking it slow?
Most of the time you get tanks in i100 and they'll either be considerate of lower level people who haven't hit the cap of that dungeon and ask if they want a full clear, OR the lowerlevel will ask and the tank will comply. (Regardless of if they've actually replied lol) So it really isn't an issue. Sure you get unlucky sometimes and SUPER COOL LEVEL 50 TANK GUNNA SPRINT THROUGH ASAP but that isn't the majority.