Valve has updated the DOTA 2 client with an emphasis on Spectator Mode. The patch has added a number of beginner-friendly features like the opportunity to check out heroes and their abilities as they’re chosen, as well as the ability to rate matches and leave comments – but also adds a couple of banner ad spaces. The banners currently invite players to check out the upcoming Invitationals tourney, but there doesn’t seem to be any reason why they couldn’t be rented commercially to help offset the cost of hosting the extensive Spectator Mode service. If it does go commercial, the advertising space will be just one of a number of Valve’s initiatives for making money from the free-to-play MOBA, some of which reward the community as much as the developer; box office proceeds on premium streams are split with tournament organisers, for example.
Source: DOTA 2 Blog