Also, in my limited experience the 30% also goes towards lots of other valuable features like access to the automatic marketing/recommendation engine. By launching your title on Steam, people will see your title in the "More like this" recommendation bars that appear at the bottom of each page. And if you like certain genres of games, Steam will just directly recommend you more titles of that genre on the home page.

I launched a steam demo of my programming game last month (you can find it in my HN profile if you are curious), and without having to do any marketing of my own built up about 1000 wishlists in a month just thanks to getting traffic from similar but more well known titles like Exapunks.