I'm seeing more and more of this stuff pop up on Steam. Especially noteworthy now since you also saw several of them going for sale during the winter sale. I can't help but feel we've opened some kind of Pandora's Box here.
So they're essentially betas that you pay for. Or even alphas in some cases. The classic notion of betas used to be that a select amount of individuals play your game for no charge and are expected to find and report as many bugs and problems as they can, so they can be ironed out for release. I think I first started to notice a change in this paradigm back when the World of Warcraft beta was going on. It was moreso about trying out a cool game for free than actually testing (although Blizzard appear to be one of the few who still take this stuff seriously).
Then things really shifted once we entered the 360/PS3 era. Betas suddenly started appearing on consoles too, but they were clearly being treated much more like promotional material and incentives that were given out for pre-ordering the game or purchasing another game (I think it was Crackdown 2 that pretty much sold itself on "buy this to get the Halo 3 beta"?). This development also neatly coincides with the death of demos.
Which brings us to the current situation where more and more of these "Early Access" games are appearing on Steam that you flat out pay for to gain the privilege of "testing". It's particularly rampant with these online survival type games that are now appearing in droves in the wake of DayZ. The usual claim is that you get to be much more involved with the development and direction of the game. But I've just grown far too cynical to take that stuff at face value. It seems very evident to me with these types of survival games that it's more about quickly cashing in on the trend and luring in players with the enthusiasm they have at the moment for DayZ-like games.
This stuff is no doubt going to spread from Steam to the consoles once publishers see how lucrative this is. I saw an Angry Joe video about this subject, and he said that the Mechwarrior Online "beta" has been going on for over a year now, with barely any of the major issues fixed, but they do have a fully functional microtransaction shop! That's probably the kind of stuff we can expect to become the norm.
The only Early Access game I've purchased is Starbound, and it seems that they're living up to their word and are actively implementing fixes and player suggestions, so good on them. I've considered purchasing a "founder's pack" for Everquest Next landmark but questioned wether it's really worth it (especially since this will be a F2P game).