While I don't really see DotA II being free, I think being free to watch should be and feel it would be a major community boost.
Two of the biggest problems with E-SportZ(Video games played for money) is gaining a userbase to play and then gaining a userbase to watch the game. Often times the best way to watch a game is right in the game itself. E-SportZ has a very large market and is highly untapped by almost all developers.
Current game or demo watching will also increase the amount of movies made, which has a massive userbase and following.
AdvertisementZ are highly untapped by the gaming industry and for ads to work you normally want as many eyes as possible. The only way ads will work for Valve and not YouTube or any other company is by forcing you to watch from the game.
The next biggest step in the history of gaming and E-SportZ would be to allow the gamers themselves to make money. Imagine your favorite player, IceIYIaN or GabeN or whoevever, to gain 1-5% of the profit from these ads. So, for example it's me(blue) vs GabeN(red). His team has red ads and mine blue ads. However Valve wants to do it really, but I'm hoping they at least look into it.
Finally, allowing modifications to make money via ads would hopefully allow their creators to "Enter Commerce". Under United States law you have no right to a trademark unless you enter commerce. This is why myself, who's worked on something for eight years lost everything. My name, my community, my future. Sure, I have all my useless copyrights, but the trademark is lost forever. A major part of any business.