Good for him. Montreal should have held on to him, but why put faith in someone you need.
Instead, put love into a mental midget because he's from the providence.
I think when trades happen, people have to understand it goes beyond player value VS player value. Of course value is into play, but there are other factors GMs consider.
In Montréal, it is well known that the franchise image is very important. Kassian was warned when he arrived that there wouldn't be a second chance. He had to grab his chance from the start, which he didn't.
What should the GM do? Tell himself "hummm, Kassian has a power forward profile we need on the team, so screw the organization's values and screw what I told him when he arrived". What kind of message does that send to other players on the team? That the GM doesn't hold his ground if he needs you in any way or form. Very bad message to send to an organization.
Bergevin was honest from the start and held his word.
That kind of an attitude is what earns respect from other GM and players around the league. This can lead to players appreciating the values of the team and be willing to sign in Montréal at some point in their carreer.
Of course Bergevin was dealing from a position of weakness when he did and when you only look at player value VS player value, he didn't fare very well. That's obviously the best he could get at the moment he traded. But the respect he got from other GMs, other players, his own players, Kassian himself (for holding his word) will probably have positive impacts in the future that are impossible to measure.