I would respectfully disagree with you. If he is bad defensively, why did Jersey use him on the PK? He was second on the team for forwards SH TOI per game behind Haula.
1.) Defense and penalty killing are not the same thing. Mercer is a good PKer but is a dog chasing cars at 5v5.
2.) Load management for stars who could PK (Hischier, Bratt) but would rather be deployed in other situations. Hischier in particular got overused on the PK out of necessity when the centers started going down.
3.) NJ's depth was depleted from injury (Nosek, Bastian), non-hockey stuff (McLeod), and the loss of other PKers last summer (Sharangovich and Boqvist).
NJ's roster was really stressed at the end of last year due to depth. With Hughes playing through a shoulder injury that left him unable to take faceoffs and McLeod off the team, NJ's center depth was basically Hischier + Haula (better as a winger) + Lazar (4th liner).
People think he's good defensively because he's a good old Canadian boy who skates hard and looks like he's hustling, the kind of young player that would make Don Cherry proud. But he really doesn't know what he's doing in his own zone - blown coverage, overcommitment, not knowing his assignments. He's physically quite weak and bad at winning board battles.
He had a -20 goal differential at ES, 2nd worst on the team and T-24th worst in the league - comparable to Huberdeau, Ovechkin, Mike Hoffman, and Brendan Gallagher.
He has a lot of talent but has really failed to develop his game since he came into the league. He plays the same way he did when he was 19. Us Devils fans have spent the whole summer hoping he will pick a lane - either get leaner, faster, and more explosive, or get bigger stronger, and more physical. Either would work, but right now he's not fast enough to blow by defenders in the way Hughes/Bratt can, and he's not strong enough to win battles like Meier/Palat can.
Right now he's just kind of stuck as an above average talent going out there and playing street hockey with no direction or identity. Maybe Keefe's structure will help him. This contract could easily be a steal, but as of right now he certainly hasn't earned anything more.