Quoting: Sabres923
https://twitter.com/jfreshhockey/status/1380931852186361859?s=21
Montour's defensive performance on the ice was obviously deteriorating by the time he was traded from Anaheim.
However, it's important to note what was also going on surrounding Montour.
Two weeks before Montour was traded to Buffalo, Anaheim's General Manager fired the Head Coach and decided to play the role as both Head Coach/General Manager.
This was the first season where the Ducks started tanking and went into the rebuild that they are still in today.
They lost a bunch of other key pieces like Corey Perry, Andrew Cogliano and Marcus Pettersson.
All of this had to have affected Montour's performance on the ice.
Nevertheless, Buffalo spent a 1st + a prospect on him when he was still under team control.
Then, he was traded a couple of years later for a 3rd when he was a pending UFA.
At the end of the day, what Buffalo and Florida were trading for when they got Montour was what he could produce offensively.
Even the thread from Jfreshhockey says "his production has been decent".
Ultimately, when you trade for someone or sign them to a contract, you look for the good things they can do on the ice while taking into account their shortcomings.
After the pros and cons are weighed, the decision is made