I know that steam would help get visibility on the game, but it's questionable as to whether or not it would be worth the effort of getting everything set up to support it. Also, if it remains a free game, there's really nothing in it for valve to distribute it, unless they change me a fee for it, in which case, where is that money going to come from?
There are a lot of things that are legal grey areas, such as some of the sound clips, which would likely have to be resolved before the game could be distributed on steam. I'm also not sure how it would work with the GPL code. I imagine the game would need some proprietary steam code added to it, which may not be GPL compatible.
In short, it's likely to be a lot of hassle that could take months of development time. That's time that could be spent on more practical features, like bots, better map formats, etc.
It also means that there's going to be a lot more work that has to go into each update, as it would have to be submitted and approved by steam, slowing down the already-infrequent updates.
