I was just thinking about this the other day as well..however,
The only way it would have a point is if it was included commercially (or if steam bought it, to provide it free for their customer base [hahahaha...yeah]), otherwise - it really wouldn't get much gas on the popularity anyways, and would gain limitations.
You can't just add a game to steam and have all the bells and whistles that the commercial products on there have. I may be wrong, but as far as I know, the 'steam updater' is only for commercial games.
Long-term steam users (back from WON) would know how ugly valve is, and sees them in their true colors. They are not a respectable company - they are drastically greedy for the extra dollar and provide a huge lack in professionalism and support. I say this because, due to that 'greed', they probobly wouldn't accept teeworlds commercially since it is/was open source - they wouldn't have enough control over the product.
Just my two cents - I know a couple people who have had their product accepted into the steam listings. I don't think steam is the way to go. Up to the devs!