I think BB sometimes rotates certain guys through the practice squad as a sort of extended shadow roster/tryout spot. So like with Thompkins, he knows the system, he's now had a couple of weeks to work with the team in practice this year, BB's had a couple of weeks to see what progress he's made over the last year, and there's depth right now at WR given LaFell is waiting in the wings as well and they don't even have a game this week to worry about. So they can release Thompkins and evaluate someone else at practice this week. If two receivers go down in week 4, they probably can still sign Thompkins again even if he ends up on another practice squad. Not riskless, he could end up on an active roster, but if they know what Thompkins is and they know its unlikely they are using him very soon and he just cleared waivers two weeks ago, why not shuffle in someone else that might eventually have more value than sixth string WR and give them the week of practice reps?
Barker and Vellano sort of fall into the same category