Coleman helps us massively against teams with a lot of small, quick receivers. Slot corner. The stuff Tharald Simon couldn't do in SB49 against Brady, Amendola and co, when Marcus Burley was inexplicably inactive that game. (Not allowing for Lane getting injured early in the game and an injured Simon sucking massively) The stuff that Jeremy Lane may or may not be able to do effectively this year.
And probably slightly better, at that slot corner part of the game, than the unfortunately injured Elliott. Elliott was a serious loss for this D, but getting Coleman softens the blow. Elliott was awesome on Special Teams and solid at corner. We'll see if Coleman can bring it on Special Teams to miss Elliott a little less.
Don't know offhand if Elliott has any chance to return this year, but if he does, getting him back late season would be awesome to have two guys to cover quick slot guys, plus/minus what Jeremy Lane brings this year, if he isn't traded.