Some people that I've spoken with have said it is. They love the old raids and pine for the days of yesteryear. With transmogrification, they hope to find groups of like-minded players to run the old content, expand their raiding beyond just the cutting-edge tier. They want to experience the game for what it is in its entirety and not focus on a single microcosm of content.
To them, transmogrification is (hopefully) the vehicle in which they'll ride to the good ol' days again.
I suspect, although don't know for certain, that transmogrification will contribute to a positive bump (or a slowing of the decline) in subscriber numbers. A commenter in The Queue asked the other day if the upcoming Raid Finder will allow for players to find groups for older raids. While it might be unlikely, given how the current Dungeon Finder behaves, a combination of that ability in the Raid Finder and transmogrification might be enough to draw back the old-schoolers in droves.
So let's hear it: Is transmogrification going to bring you back, or at least keep you around longer?