Edmundson is a good example of the "advanced" stats people ignoring context.
They will look up God knows which stats to argue that teams are bad when Edmundson is on the ice, ignoring the fact that part of the benefit of having Edmundson on the ice for 18 minutes a game is that the rest of the team will play better in the other 42 minutes.
Bergevin has explicitly said that Edmundson has been brought in to be Petry's partner. So just that there helps the team, as it pushes Romanov, Kulak, Mete, Juulsen, to the third pairing, where their relative effectiveness will be greater. It may also help the three younger guys better transition to the NHL . Whenever a team adds a player, it may get more benefit from other players being pushed down the depth chart than from the added player himself.
The other thing is that Edmundson is a good PKer, whereas the Habs are not good PKers. They were 19th in the league last year, that's bad. Having Edmundson play on the PK will improve the PK, improve the team, and improve the Habs at 5on5 and at 5on4 because the other D will be more rested.
Defensemen are not like forwards. There are only three defense pairings. That means that it's very, very hard for a team to carry a defender like Victor Mete who is a 5on5 specialist. Every defenseman needs to be able to contribute to either the PK, the PP, or both.
Bonus: Other players are less likely to ram into Price if Edmundson is on the ice.