I see the logic behind the deal, but I don't like it for the Leafs as it weakens the 2nd line considerably..... Roberston is unproven and playing Jarnkrök in the top 6 makes me luke warm.
Sure, Dumba is a massive upgrade on the back end for sure, but I think this exposes the first line to playing much tougher competition on a regular basis, leading to decreased production there as well.
Just don't see this one as a case of addition by subtraction.