Buffalo has a good group of prospects that can turn into an effective 3rd line or overall secondary scoring...eventually. If we want something like that now, Mittelstadt should stay put and be at least one reliable player for that purpose. Hopefully Greenway and somebody else can make a useful 3rd line (assuming Quinn and Peterka improve also). Trading Mittelstadt, even if for a 2nd pair RHD, is tanking that idea, as replacing him puts even more question marks into 3rd line effectiveness, especially defensively.
Let's keep the small bit of good depth that we have until a couple prospects get their NHL game together. There are plenty of other assets to trade that make more sense.