I actually disagree with both companies on this issue. Blizzard says that Valve shouldn't get to own the DoTA name, and while they are technically correct, DoTA is no longer just a name. DoTA has now become so widespread in the culture now that DoTA is a completely separate genre now, and you cannot own a a genre.
A rough equivalent would be if the makers of Wolfenstein 3D thought the makers of every other FPS ever made were dicks because they copied Wolfenstein's gameplay. That makes no sense because the genre has evolved so much since then, it is almost unrecognizable.
So, TL;DR Valve has every right to make a DoTA game, but trying to trademark something that has evolved to basically become its own genre is kinda stupid. Also, Blizzard should just chill.