First, to be up front, I think cross-faction Real ID is a fabulous idea. This is motivated by pure selfishness on my part. Half of my Real ID friends are Horde, the other half are Alliance. To a degree, this is ameliorated because we have a lot of alts on both factions, but it's not totally addressed. Some of my friends have no alts; others only have alts on the same faction; and still others have one main they dedicate 90% of their playtime to and a host of alts who barely make level 20. We'd easily be able to put together a raid for any of the content in the game, save for that faction wall. We can talk to each other and put together smaller groups fairly easily to steamroll old raids, but doing content like Blackwing Descent or Firelands is arduous.
I do understand that not everyone would be on board with this, and there are good reasons to be discussed. Cross-faction Real ID raiding would not only be a huge change, but it would also cross a line Blizzard has managed never to deliberately cross in all its time of allowing new services like faction and server transfers and character customization. Sure, your Alliance warrior can join a Horde raiding group now, but in order to do so, he or she must become a Horde character. There are no humans raiding with orcs. It's been the case in the game since launch (to the point that Forsaken players lost the ability to speak with human players to preserve it) that the two factions are separate and cannot group together at all. To change that, even for just Real ID friends, would be a huge change in the game.
That being said, here are my reasons for cross-faction Real ID raiding.