I am not a big supporter of it. The current system ain't broken, so don't fix it.
However, the concerns that this will allow mediocre teams into the playoffs is overblown. Rarely does the 4th seed has a better record than the 7th seed. So the added team is actually better then some existing teams.
For me the open question is will this improve or hurt the quality of week 17 games. With another spot you could have more meaningful games. But it is also possible a team that clinched their division but is out of it for the one seed will rest starters rather than play for the two seed. Likewise clinched wildcard team might rest players considering the difference between 5 and 7 unimportant.
Not sure how this would have helped the Dolphins in recent history. When was the last time Miami was the 7 seed? NEP would have benifited if this was the system in 2008.
I think this opens the door for 8-8 teams to make the playoffs, this year alone would've saw a few make it with just 8 wins. That isn't playoff football to me. The system is just fine the way it is now.