Well, on my server DRKs aren't in real high demand (most of the time). I have 2 HL DRK friends, and they sometimes wait for hours for party invites.
PLDs, I have seen tons of them aswell, but I'm sure they are more needed in parties, because there are less awesome tanks out there than damage dealers.
You can always replace a DRK with a DRG, or a SAM, but when looking for a Tank, you have 2 options, WAR, or PLD, and most prefer PLD.