The problem is that 129F is a borderline safe temperature at any length of time. 3-4hrs is closer to what you want to kill _all_ the salmonella and e coli, but 2hrs kills enough that it's very likely to be safe, and 129F isn't a sufficient temperature for any length of time to kill clostridium perfringens and other similar bacteria, resulting in spoilage occasionally with a greatly increased likelihood the longer you cook it.

Separately, yes, you do normally sear it afterwards. It's for flavor/appearence/texture reasons though, and a number of people recommend mildly chilling it (say, for 15min or so) first so that you don't overcook any of the interior from the sear. The pan should be much hotter than a lot of people expect.