I thought this was cleared up ages ago.
Valve's game is not "Defense Of The Ancients 2," it's "DOTA 2." No implications attached and, thus, having no connection with the old game through name or title. They're not claiming a trademark over "Defense Of The Ancients," they've claiming it over the unnoficial name of "DOTA" (Which, by the way, is not "DotA." DotA is an abbreviation while DOTA is a word in capitals).
However, I will make a note that the argument seems to have changed. Blizzard is no longer stating that Valve can't have the name due to its use in the community, but rather that they own it. This is something I see to be ultimately wrong - it's like saying that every game which runs on an engine belongs to the people who made said engine. Warcraft 3 was used as nothing more than an engine for what started as a Hero Defense minigame into a full-blown Genre.
Also, looking between Blizzard DotA and DOTA2, in terms of content rights DOTA2 has more content similar to that of the original DotA, while Blizzard DotA is made up of Blizzard's "Greatest Heroes."
If Blizzard wants to say that Valve can't use the name DOTA, fine. But Blizzard shouldn't be able to use it either, if that's the case.
I find it funny that Valve hasn't launched their own campaign against Blizzard, going by how much they complain about it. Isn't Blizzard DotA a mod and, thus, not a full game to be trademarked?