I wasn't surprised Dean didn't like Ketch punching the vampire - shades of Gordon, really. There's hunting and then there's taking sadistic glee in the kill. Ketch seems like a more sophisticated honey-trap to get Dean back to his more violent ways. I think Dean sees himself that way a lot, especially after MoC.

I think Mary has a reputation as the best Winchester for them because she has the hunter pedigree but isn't associated with any of the major occult apocalyptic things that the boys had to deal with, so she's got the skills without the apparent black marks. That and she was "smart" enough to willingly work with them.