Sorry this reply is so late...but I just noticed it. I had to surf all the way back to July, but....
I think the idea/deed is cool. I can understand why he'd put it on his back. There would be less distraction there to anyone who'd actually want to read his vows. Put it on his chest, and rather than a person looking him in the eye to speak with him, they'd be busy reading and then just looking back and forth. The back is more of a 'wall to be painted on' than the chest, with it's nipples and other assorted hills and valleys. The back is a flatter surface.
A tattoo is a commitment to a moment in your life. The thought that the marriage could fail is not nearly as important as the fact that right now, at this moment in his life nothing is more important than the meaning behind those words. Regardless of what the future brings, he'll always remember how he felt at this time in his life.
I'm speaking from experience. I've got 10 tattoos, all telling the story of my life, chapter by chapter. I don't care if they should sag, that never was nor ever will be a concern of mine. The art is there because of how strongly I felt about periods of my life.
A hundred words on his back...must have taken a bit of time. Pretty cool, I say.