Other classes have to deal with similar limitations. Having multiple Shaman in a raid means you can have more totems up at once. Multiple Paladins give a variety of Blessings and Seals. Bringing two Holy Priests, however, doesn't give you any additional buffs, just a second healer.

Death Knights, in addition to our reputation as "Deathknoobs," share the Priest's problem in that we don't provide any significant buffs beyond Blood/Frost/Unholy Aura, which is spec-dependent, and Horn of Winter, which is basically the same for all DK's. If you already have an Unholy DK DPS in the group, a second Unholy can't offer anything unique.

High-end raiding guilds are beyond the scope of this article, but I'd say that standard application rules apply for DK's, except more so. Show the prospective guild that you're not just trying to ride their coattails by buying/earning the best gear you can find, then gemming and enchanting it as well as you can possibly afford to. Before logging off, change into the gear that you want the prospective guild to see in your Armory profile. Be prepared to have your talents and your Achievements scrutinized; they want to make sure you know what you're doing.