You can't assess his kicking based on touchbacks alone. His best game for us was the Jets game we played in Detroit. He could have kicked every ball out of the back of the endzone, but he didn't. He boomed high, arcing kicks with air time to completely neutralize Harvin. I don't think Harvin ever got past the 20 in that game, and I'm pretty sure he got stuffed inside the 10 a few times because of those kicks.
I'm not lobbying for a kickoff specialist, just pointing out a situation where you would think stats could tell the story, but they really don't.