People like to crap all over Stewart, but him plus another quality asset or two really is pretty good value for a UFA Callahan. You get a player back who can step right into Callahan's spot without the dropoff being too bad, and he's locked up for another year. You also get some assets for the future. Just like Pouliot and Brassard, Stewart is inconsistent. That doesn't make him a terrible player or lazy. If he can put it all together under a new coach, you have a possible long-term asset. If not, you can still flip him to another team next deadline for a decent return.
Obviously, it depends on what the Blues would be adding on top of Stewart in that deal, but if you can get one or two really good additional assets, that would be a smart, shrewd deal.