Technically speaking: Cortex Command, Overgrowth, Minecraft, and Natural Selection 2 have the same preorder business model going on. The only real difference between them is that in Cortex Command's case the money it takes to buy the game goes up the closer it gets to release and in NS2's case if you spend near the full price of the game as opposed to minimum pre-order you get in to the Alpha builds, tools, and whatnot. NS2, Overgrowth, Cortex Command, and Minecraft all offer the current alpha of the game in exchange for a pre-order and in all cases the pre-order you make counts as the full price of the game when it's released.
So, if you don't see the point in buying Minecraft, you wouldn't see the point in buying Cortex Command, Natural Selection 2, or Overgrowth at this point in time. To those who said that there was no point in buying an Alpha, would you buy any of the other titles? I'm just curious.
As a miscellaneous note: "Alpha" doesn't mean 'void of features'. Alpha just means (in almost all cases) "before feature lock"; which can be interpreted as 'not all the features in the game have been integrated/completed'. For example, Star Ruler is roughly 65-70% complete at the moment (diplomacy, trade, combat, easily adjustable and procedurally generated galaxies, interstellar phenomenae, fleet management, planet management, research management, completely customizable ship creation/importation/exportation, mod and map selection, easy mod/level/scenario creation, star differentiation, a particle system editor, a model viewer, localization support, a pinned object system, a multi-mode object information window, ship orders automation, more than 50 subsystems for ships, a few tracks for our soundtrack, almost all of our sound effects, a full newtonian movement system, a robust contextual command interface, most of the tutorial, multiplayer support, and lots of other little bits and bobs) and we're
still considered to be "in alpha". The game isn't buggy, it doesn't crash except under extraordinary circumstances, and save/load/import/export works flawlessly. But we haven't put in -everything- just yet so we can't really call ourselves a Beta. Beta is when you've got all your stuff complete and you're polishing for Release/Gold Master.
Just saying that when people think 'Alpha' they probably think of the game being little more than a prototype or a tech demo; it's really more of a case-by-case basis in reality.