Allen is underrated here. He had two bad seasons with the Blues and the last one of those was when Binnington took his job and lead STL to the SC. That being said, in his last season with STL, his save percentage was back to 93% and he was good for them in the playoffs. He started last year very well with the Habs and then his save percentage fell.
The reason is simple : he is a great backup, but he does not have the ability to play a lot of games. The more he plays, the worst his performances are. The question is : now that he has been rested proprely (because of injury), would a team take a chance on him for a potential SC run this year as a 1A/1B and as a backup next year. And if so what is the price of this ? I don't think WSH is super happy with Samsonov but I don't think they are the ones really desperate for a goalie.
If any player is traded between WSH and MTL, I believe Lehkonen would make more sense (but I don't want to enter in the debate about his value here
)