Demario Davis will enter his fourth season in 2015. After playing sparingly his rookie season, Davis became a starter in 2013 and has not missed many snaps since, totaling over 1,000 in both of the last two seasons.
Davis shared the inside linebacker position with David Harris. Davis' tackle total was 116, a robust number. That tied him for 13th in the league. That might be overstating Davis' effectiveness. He made a lot of tackles, but he wasn't an impact run defender. He didn't make a ton of plays to ruin rushes that required something special. He only had five tackles for a loss, and PFF rated him 24th out of 35 inside linebackers in terms of tackles that result in a failed offensive play per snap.
Davis also is not a particularly strong pass defender. There were hopes when he was drafted that he could use his athleticism to eventually become a good cover linebacker. It hasn't happened. It doesn't look like he has great man to man skills, and in zone coverage he struggles at times identifying the proper depth to drop.
Three years in Davis looks like a functional starter, nothing more, nothing less. I think if he is the eighth or ninth best player on your defense, you are happy, especially on a cheap rookie deal, because he can perform in a limited role. If he is your third or fourth best player, you aren't happy because he isn't going to be able to carry the load.
Davis is very affordable. I take no issue with him continuing to start. I hope the Jets can fill out the defense around him so he take a supporting role that suits him.