Utilizing the Dungeon System to fill one or more spots in your group will grant you the Luck of the Draw buff, which increases your damage, healing, and base health by 5%. It doesn't matter if the dungeon is random or not, or whether or not your entire group is a PUG -- you still get the buff just for utilizing the system to fill at least one spot instead of spamming the (now) always-open LFG channel.
Blizzard's really pushing the Dungeon System, and after playing with it, I can see why -- it rules -- but some Luddites might be harder to convince, and I suppose this buff is Blizzard's way of saying "Sure, but would you do it for ... Mr. Lincoln?"
Patch 3.3 is the last major patch of Wrath of the Lich King. With the new Icecrown Citadel 5-man dungeons and 10/25-man raid arriving soon, patch 3.3 will deal the final blow to the Arthas. WoW.com's Guide to Patch 3.3 will keep you updated with all the latest patch news.