Tanks and healers tend to be rare in general due to the importance of the role: A lot of players don't want to take responsibility, and prefer a role where if they half-ass it nobody minds. Ironically enough though, this does actually mean skilled DPS are probably the rarest thing about. Usually you can take any random tank or healer and assume they will do their job adequately. They signed up for the responsibility, and usually care enough to learn how to play their job well.

DPS, on the other hand, as I mentioned earlier attract a lot of players who don't like responsibility. Your average DPS player will slack off, expecting the other DPS, the tank, and the healer's damage to carry them through the instance. However, a lot of content does actually require DPS to bring their A game, and on top of that actually playing a DPS to the best of one's ability is harder than a healer or tank IMO. These two factors combined mean that DPS who push their damage to the best of their ability while still executing mechanics are rare and precious.

So, I suppose it boils down to whether you want fast parties (tank or healer) or wish to have better chances at smoothly completing content (DPS)