I think Valve is trying to slowly divest itself from Dota properties. Basically they realize that Blizzard doesn't actually have much of a legal case right now as the game is still technically in beta and is constantly changing. But what they don't want is when they release, Blizzard instantly sues them and they get locked in a years long legal battle even if they have a valid case. The worst thing that could happen would be Blizzard wins an immediate injunction against Dota 2 and they have to spend years to repeal it. Some people might argue that Blizzard wouldn't do that, but I think all anyone has to do is look at Blizzard actions these past few years and you'll get all the evidence you need.
Anyway, basically what they need to do is prove that they aren't riding off the intellectual property of Blizzard, even if they are. So for example, say they get sued (I think they will), they need to show the judge that they've been deliberately spending resources and time to break away from Warcraft models and names and things like that. Suppose they didn't do that, even for minor things like Obsidian Destroyer, then Blizzard would have a much stronger case for saying that Valve merely copy-pasted all its ideas... right down to the similarities of the names.