Bornakk says balancing both Dual Wielding and 2Handers in each talent tree has been exceptionally challenging, and I think we've seen that firsthand. Talents aimed for one build make their way into another, and things quickly get out of hand. Turning one of the trees specifically into the Dual Wielding tree (whether it be Frost or Unholy or whatever) is something I can get behind, even if it runs counter to what they've been trying to do with the talent trees since the Wrath beta.
Allowing all three talent trees to tank and DPS depending on point allocation isn't that much of a stretch, but I think that trying to make them great tanking trees, dual wielding trees, and 2H trees does stretch things a little thin. Having one dedicated Dual Wield tree along with dedicated 2Hander trees would probably make the individual specs stronger overall in the long run.
I admit that it would probably suck to be the Death Knight that loves the 2Handed Frost play style and gets that pulled out from under them because it became the dual wield tree, but I still can't help but feel it would be better for the overall design of the class. Assuming they can keep Dual Wielding and using a 2Hander roughly equal as far as DPS goes, anyway.