Belichick made Langi the highest paid UDFA in the entire NFL this year. Obviously, he has plans for him, and the idea is that he wouldn't expose him by cutting him in hopes to make the PS. He is also more athletic and offers more on STs.
McClellin isn't in the mix as much since he is so versatile, despite my thinking that his cap hit is too high.
This is hardly about "rooting for the underdog." To me, the additions of Langi, Harris, Rivers, and Wise put a guy who's predominantly a 3rd down rushing specialist squarely on the bubble...and that's not even bringing up the point that HT will likely be rushing more, as well.
Of course, this is under the assumption that they'd keep Roberts, and that there won't be any injuries. Like I said in my post, there's always injuries at the position, so we will likely be re-assessing our LB depth charts again in 3-4 weeks, anyway.