A great deal of Valve's public image and reputation, which is a pretty big deal to everyone involved (fans, customers, Valve's heads). If Valve let Blizzard Dota pass by in the media without incident and then suddenly started raising a ruckus down the line, it'd look pretty rank.
They can't just ignore it if they do have DotA trademarked. Blizzard DotA would definitely dilute Valve's claim.