McClain was overdrafted a year ago. As a 6th round pick he could have made sense for the Panthers, but they were desperate for DT and they took him way too high in the 3rd round.
He did not play well last year as a rookie, and the fact that they just cut him after 1 year tells you everything you need to know about him. It would be the equivalent of us cutting Akeem Dent this year. At best, I thought coming out of USF, he could have a role/career similar to Vance Walker. But at this point, he's not better than Walker, and no I don't think he's better than Robertson either.
Here's my scouting report of McClain last year: viewtopic.php?f=46&t=14755
As for Carpenter, that guy has gotten about 5 chances in the NFL, and never worked out with any one of them. Why should the Falcons give him a 6th one?