Oh no, how dare someone suggest that each role bring something of critical value to the table. If that's not achievable, get rid of your trinity design and make everyone a dps. Either is fine.
Tanks have very little impact on gameplay at the moment. Positioning and movement is largely done for you. Raid damage is more relevant than the damage that your tank is taking, even when there's a *dramatic pause* tankbuster. And you often have a substitute tank when one dies.
A tank death should result in a near instantaneous raid wipe. The primary healing challenge should be keeping your tank alive, rather than raidwides. Your tank's mob movement and positioning should always be a essential factor in making raid dps checks. Make tanking into a linchpin role, and the people who actually want to play tank will play tank. Make dps into a linchpin role, and everyone will just play dps instead.