Allen has 2 years left on his deal, makes about the same money as Smith and is almost 10 years younger.
I'd like to think a Smith for Allen trade would make sense to both teams but the Flames take on the risk of him not regaining his form for 2 more years.
Might be a risk worth taking, and the Blues might just want him off the books. That being said I don't know who St. Louis has in waiting that they can bring up but they may be a more attractive landing spot for a guy like Bobrovsky than say Calgary would be. But now I'm way off topic.
