Good DPS and Tanks

While rogues and hunters do not provide many buffs or group benefits, most groups bring them for their DPS numbers. However it is not uncommon to get a Hunter or Rogue in a group, have them do horribly low DPS and therefore replace them, or simply never invite them to another group. With the options available to buy 4story Gold, there is no need to settle for anyone sub par. The same applies to Death Knights right now. Sure they are very good DPS and Tanks, but there are a million on the servers. If there are already 3 in a group, do you really want another? Would not it be better for everyone in the group to bring a different class to get a different buff and give the three in the group a better chance at any gear that drops?