Pierce has only lost to the cream of the crop at 170 lbs, including close fights against Josh Koscheck and Johny Hendricks. While he lost pretty clearly against Jon Fitch, he did manage to wobble him a bit in the final round. Pierce's last fight against Seth Baczynski was far from pretty, but he won by unanimous decision to give him his 2nd win in as many fights.
Mitchell saved his UFC job by sending Simeon Thoresen out of the promotion with a unanimous decision nod at UFC on Fox 6 in Chicago. He previously came up short against T.J. Waldburger and Paulo Thiago in his other UFC bouts.
It's a rather odd fight to give Pierce, who was briefly ranked in the UFC's top 10 this year, but I guess when you have the reputation of being a "grinder", the matchmaking standards change.