Linebackers should have the bulk of the tackles. Hell, Ellison has had double digit tackle games this year. Does that make him better than mediocre? Watch any game and you'll see 'backers making explosive or disruptive plays. The Bills do not. That includes Poz, who may be a serviceable part of a starting linebacker corps, but should never be the focal point based on what he's shown.
Mitchell would be better in a situational role when blitzes are called. Though pretty unreliable in the open field, he has shown he can make big plays on occasion, and seems to be the closest the Bills have to a starting linebacker in terms of ballast. I don't know if what he earns jibes with the idea of him as a part time player, but I wouldn't be reluctant at all to show him the door at the end of the season if the club decides to fumigate.
I can't for the life of me figure out why they didn't take that Cincy USC backer in the 2nd round.