Steam, even with Greenlight, is intended to host a curated selection of games. That is part of (or at least it traditionally was) Steam's value to consumers, but it has the side-effect of introducing bias against small, rapidly-developed, prototypical or otherwise less-polished games.

Here's a niche: Some people make neat game prototypes for competitions like Ludum Dare with a surprising amount of potential. Not everyone can afford to quit their job to turn their prototype into a well-polished product that can succeed on a popular app store or be crowdfunded (nor is every idea mainstream or even good enough to warrant that), but they are probably capable of at least cleaning up their prototype a bit and adding a decent set of levels or other content.

If someone makes a generic but fun game for Ludum Dare (say, a pixelated-platformer with a gimmick), I'd probably be willing to toss them a few bucks, mostly out of good will, for a more-polished but still quickly and cheaply developed successor. Whether they use that money towards developing an iteration that can succeed on a popular crowdfunding site (which are being quickly taken over by marketing experts) or as pocket change is fine with me.

This use case could be even more well supported by a Bandcamp-like donation system (which is what "pay what you like, or nothing at all if you are aware that you are allowed to enter $0" effectively is).

I'll end my reply by wondering aloud if a new generation of freeware games would ever be viable -- high-quality niche games released for free (and possibly open-sourced), and supported by a no-strings-attached donation scheme.