Well one of my reasoning was that steam is so important because developers made it so important.
Currently it's a monopoly on the PC and I don't think that is good for developers.
You're wrong, actually.
Steam started off as a distribution platform for Valve's own games. Then they started selling other people's games, and that made them enough money that they shifted to focusing on that and releasing games less often. But enough developers found that Steam brought value to their games that word spread, and it became the defacto distribution platform for digital download PC games.
Steam adds a ton of value to your game, for the simple reason that during their sales older games that are long past their peak selling periods suddenly come into the spotlight again, and make their developers substantial amounts of money again. There's a really good piece on Gamasutra from the Bastion developers explaining just how good Steam has been for them, sales wise.
Not only that, but it gives you the peace of mind that you can make your game and when it gets picked up by Steam, you can have your marketing/PR blitz, and that will generate you sales. Then everytime there's a Steam sale, chances are your game might get featured, and make you more sales, and so forth. And you don't have to worry about having a secure website to deal with people's credit cards - or contracting separately for someone to do that.
Of course, I have no personal experience, but the reason people release their games on Steam and such is because the 30% you have to pay them compensates for all the stuff they take care of for you that you no longer need to worry about, allowing you to collect money for your work, and focus on working on your next game.
As for wether 30% is too much or not, it might be, but it's become the de facto standard for most digital download platforms. If you don't like it, no one is forcing your hand. You could always release your first game on Steam (if they pick you) to help get you an audience, and then release your subsequent games on your own website, and promote it to the fans you have acquired from Steam.
As a developer, you have options. It's part of your job to weigh those options, find what works for you, and if nothing does, figure out an alternate route that does.