The problem is not that steam allows or doesn't allow open source games, it's that their DRM (which is required to work with steam) is not open source and thus incompatible with gpl code.
Not true. The Steamworks DRM is not Steam! The DRM is optional. Steam is the distribution network. For example, Gish is GPL licensed and available on Steam. Steam is hardly anti-open source. The upcoming Steam OS, for example, is open source, Linux based OS.
However, Steam Greenlight costs money. A one time $100 cost, to be exact. The submission fee is supposed to keep non-serious entries out. Once in Greenlight, gamers essentially vote whether or not the game belongs in Steam. If approved, the game is moved into the standard Steam library.
Presence in Steam does not stop the game from being distributed elsewhere. Nor does it change the license requirements. Steam can simply provide an alternative method of distribution, including easy setup. The downside is extra work in distribution (for new releases, anyway) and the $100 submission fee.
It is also unclear how pricing would work. While free products exist on Steam, Valve only says:
3. Who sets the price for my game on Steam?
Pricing is very title specific, and we've got a lot of data and experience to help you decide on what the best price is for your title. We'll work with you to figure out pricing.
It is unclear if it is possible to explicitly choose making the game free. Not that the game has to be free, given that there's a submission fee for Greenlight. A fee would help recuperate costs, and could even be used towards MG development. Since submission to steam does not require a change in licensing, the game remains GPL. It can and should be distributed freely from other sites. A separate Steam version would make the game visible to a very large audience of gamers (Steam has more than half of the digital gaming market). It's also an easy way to donate to MG, since it allows players to pay with their Steam wallet and any payment method that Steam accepts.
So the only real downside in my opinion is that submission fee. And that's actually kind of a big one. $100 is a lot for a small, free game. It's up to debate if Steam would even have a noticeable impact on MG.