Since the drops are different
Wrath of the Lich King also brings along another step in the pro-casual shift that was heralded Lunia gold along with Burning Crusade. If you will recall, BC cut raid sizes down from 40 to 25 in an effort to make raid content more accessible. WotLK further improves accessibility to raids by mirroring every 25-man raid with a 10-man version. This means that there will not be any content exclusive to high-end raiding guilds; except, of course, the loot, since the drops are different in the two versions.