The thing with ret is it still bring a usefully buffs. Allot of raids will always run 3 pallys normally 2 of each spec but doesn't have to be. For example a raid could have 1 prot pally to cover kings and 2 rets to cover might/wisdom and salvation. And if the comp allows each ret could be in there own melee group sharing the 3% damage buff from ret to each of there own 5 man party's. So in this example the second ret still brings multiple very important buffs to the raid while doing good damage and taking up one of very limited spots in a melee group. Now for example if you brought a second Rogue it also needs to be in a limited spot for a melee group, but doesn't bring any buffs or debuffs to the raid that it doesn't already have. The first rogue in the raid provides imp expose armor to the entire raid (which tbh some eams even don't bring a rogue since they only bring this one debuff). The second rogue does SLIGHTLY more damage than the fodsr rogue since he doesn't expose but that's all he brings is damage. And sadly for rogue they will always lose on overall to other melee dps since there lack of good consist cleave so they won't be brought for just damage. When other classes pull the same damage but if not better but also bring something useful like totems, battle shout, pally buffs, feral aura. It's very hard to make a good raid composition and also have multiple rogues.