Course it makes sense, since Garrosh is leading the "Horde" now, he's gonna join up with any "real" orc he can find, and nothing screams "Ork! Ork!" like a bunch of red-skins bending dragons over to ride them.

If he makes it through Cataclysm alive, I expect he'll recruit the Blackrocks as well.

You should probably consider that the Dragonmaw in Azeroth probably haven't had any contact with the Dragonmaw in Outland for a long time. Also consider that the Outland Dragonmaw had been given the blood of Magtheridon, turning them into Fel Orcs, while the Azeroth Dragonmaw haven't had any contact with demon blood in a while.

Maybe its an excuse to let us make orcs with red skin.

I have an easier way to make it make sense. "With the death of leaders Nekros and Zuluhed, the Dragonmaws were crippled at the top. Combined with the rise of the Red Dragonflight back into prominence, the Dragonmaw clan has decided to cut their losses and rejoin the Horde."

Simple and no plot holes whatsoever.

Or what about the fact that the Dragonmaw are without question the best at fighting and subduing dragons. Garrosh finds some, and gives them some options: rejoin the Horde to regain your honor, or we kill you. Simple enough. I'm sure you can guess which option they chose.

Besides, what would be a better way to make the Dragonmaw atone for their actions than to make them fight against their former ally and employer while simultaneously helping their former victims? Glorious.