The Red Sox are more or less looking for insurance against really cruddy numbers v. RHP by gomes and napoli. Carp is one of the insurance policies. He does not even have to be above average v. RHP to be a useful part-timer. He has to beat out Nava and / or Overbay, and he's only going to get 350 PAs v. RHP.
At this point, 2011 AAA looks like the aberration, but when he came up, he was still a .345 wOBA guy. He's a career .319 wOBA in MLB away from Seattle, and a career .321 wOBA v. RHP. Close to average, maybe not even average for a corner guy.
This looks more like a nice potential dumpster dive, see what he has, type of move. Doesn't cost prospects, doesn't block guys. The reverse split is not helpful, but an average wOBA v. righties could be useful. I think we are in basic agreement that it is a stretch to think that this guy should have gotten much more back for Seattle.
As for the 10%, all I claimed was that the AAA walk rates were fairly steady, around 10%, rather than bouncing around. OK, not spectacular.