Why would MTL ever make that trade??

Well,

1) Habs need a middle-6 Ldman, which Dillon is
2) Karlsson played with Dillon in SJ for the full year when he was healthy this year too (except for on the PP and sporadically with Vlasic), and for the most part they flourished and played great together except for the POs when EK65 played on 1 leg
3) MTL gets continuity with a Dillon/EK65 on MTL's top pairing to start the year.
4) With both EK65 and Weber, Habs aren't keeping Petry too
5) Habs need a reliable non-expensive veteran backup too, so Dell solves that.
6) You can tweak the prospects/picks, I concede that, but Jeremy Roy was the first pick of the 2nd round and had a tremendous upside until a leg injury brought his stock down but he's been progressing back to that level and is from Quebec City, so Habs would be glad to have him.

Omg come on Dell is worth nothing. Lindgren will be Price back up, if we really want another back up goalie, there is planty UFA. Josh Brook Noah Juulsen Cale Fleury are all RHD better than Roy. Kulak is better than Dillon, Mete-Kulak > Dillon. MTL will get a top prospect + 2rd pick or 1st rd pick + B prospect for Petry.

Drop the 2nd and Mtl might think about it, although I doubt it. Petry is better than Dillon, more than worth the cap difference, and is under contract for an extra year. Dell has no value in this trade, at his cap hit he might actually have negative value considering the season he just had. Roy's only value is his home province, Mtl have a decent supply of RHD prospects, so he is basically worthless in this trade. Honestly I don't think the Sharks have the right pieces for a Petry trade.

Exactly. Jeff Petry carried the Habs defense last season and consistently played 20+ mins per game. Petry is a #2 defenseman on many teams and isn't slowing down. He has a lot of value