It's a fact of life. Burning Crusade is about to become obsolete. All those rep grinds, all those exalted purples, all those dungeon keys are just so much pretty little bank space wasters. Sure, gear lasts a little longer into Wrath, but for the most part, we're still going to be leaving it all behind for the new environs of Northrend, while Outland becomes a bump in the road to 80.
For Death Knights, this is probably still true. We're going to want to rush to get through Outland so we aren't desperately trying to find groups for Utgarde Keep while everyone else is getting Kel'thuzad on farm mode. But that means we'll have only the bare minimum of faction, no grind, no turn-ins, no purples. On the whole this is probably pretty much OK, since we'll be able to get good gear right out the gate in Howling Fjord and Borean Tundra. But is there still a reason to grind some of the Burning Crusade factions, or even to pay attention to them while leveling up?
Let's look at some of the factions you'll probably come across while speeding through Outland and what benefits they can offer the leveling knight.