[minor nijja, while I was away from my actively editing machine...]
Yup, pretty much the whole point, as far as I can see is the "sell on Steam" bit. I understand (IMBW) that there's free games available on Steam, but I imagine the business plan is that these are loss-leaders to get people to look around for Payable ones, and possibly even accepted on the basis that a really popular free game might entice the publisher to put the next version up as a micropayment-payable one, or even a more standard 'macropayment' one as they think they can compete with the Big League games and recoup their development costs.
Toady has an outlet from which he 'sells' his wares. The 'selling' is by indirect donations (indirect to the downloading and playing process, that is), and the provision of his game(s!) gets covered by the infrastructure he's already invested in (and wishes to continue to, in running costs). If an indy developer didn't want to set up (and continue to run) something Bay12ish, then perhaps Greenlighting would be an option (among others), the only advantage Toady might get is the increase in visibility.
(But, at the risk of presuming the state of things, if he's happy with the current level, what with all of us knowing about it and getting an article in Time/wherever it was and countless other places popularising the game, then what would be the point? Maybe when it gets to Beta or even late-Beta stage he might want to re-evaluate and run ads on every cinema screen in the Western World and quite a few of the others, but by then Greenlight will probably not be the platform anyway.)