The thing to remember here is that the average 6th/7th round pick is probably lucky if he makes it to the practice squad. For most, the realistic upside is "maybe he can play some ST" (and yes, rarely Edelman or Brady happens)Yeah, I think these are home run swings with a low percentage chance of working out, but the cost is also minimal. I don't think they should stop trying just because several of them haven't worked out; it's not like it's costing much, and if one or two guys pan out it's going to be worth it..
With a former 2nd rounder, you're at least sure he looked the part at some point, so the realistic upside is higher.