If you want to improve your team or change the dynamic, the best and sometimes most painful way to go about it, is to move an asset you do value highly because it assures you're going to get appropriate value.
If all GM's did was trade players they didn't want...then it wouldn't be too much fun would it?
For me it's more about his poise with the puck, his skating ability, shot and PP presence....these are elements that are missing on the Habs so I think he'd bring a lot from that perspective.
As are the Flyers
I will say this first. I'm very interested in adding Gostisbehere to our back end and for the very reasons you mentioned. However, I'm not going to ignore the reasons why they would consider trading him in the 1st place.
We need to be careful with trying to improve the team through free agency and trades. It can go either way in terms of it working out or not. Knights traded a few picks for Tatar and it didn't work. They traded him in a package deal less than a year later and retained on his contract.
I do think Gostisbehere fits with Weber or Petry and he helps our PP. However, I also think he makes mistakes with his creativity that might result into something we are not predicting today in terms of popularity. The minute he stops producing offense or has a span where he struggles to produce like Drouin, he is no better than Reilly.
If the price is right, I'm in and willing to swing a trade if the parts we use are some of Byron, Shaw, 2nd's, Ylonen, Ikonen, Olofsson, McShane. I am a bit hesitant to trade Shaw though cause he worked so well with Domi