Do people think Tank players are just born knowing they want to play tanks? Sometimes all it takes is an enticing tank class/job to get someone to try it and maybe they'll like it. I certainly didn't start my MMO days out playing as a tank, and I thought I'd hate the role until I tried it one day in another game and found I liked it. I know of many people who were career DPS or even healers who tried out DRK when it came out. These things are only good for the health of the game, as opposed to the likelihood of tank numbers going down if they get stuck with the same options for years on end or are given unoriginal/uninteresting new jobs.

You want to keep your tanking (and healing) population healthy in any MMO that relies on the trinity. Because without those roles there's no taking on content. Without them making duty queues pop many more players will become tired of the waiting and leave. The game needs tanks and healers.