Healers are more needed in a general sense. You can get though a three man FP and heroics without a tank and that is when a healer is definitely needed. Tanks however obviously have their place, they are the ones who minimize overall group damage from the environment and opposing players. You don't NEED tanks, but not takin them is about equivilant to shooting yourself in the foot and then breaking your ankles as you crawl for a jar of peanut butter, entirely possible to get that jar of peanut butter, but why the **** would you try other than to challenge yourself? Not having a healer is like adding a tiger in front of the fridge to get the same peanut butter.
In the most literal sense, this is true. However, the OP is obviously referring to group finder pops, in which case it will ONLY pop with a tank, a healer and 2 DPS.
In that scenario, far less people queue as tanks than the other classes. There are a variety of reasons for this, but the 2 most obvious are that a tank requires a separate set of gear whereas most DPS classes can still heal effectively with their DPS set, and tanking tends to put you in an assumed position of leadership, something some people wish to avoid.
If you want fast pops, trust me, play a tank.