It seems more like they wanted to make the rounds on various blogs and news sites which will inevitably display something along the lines of "Minecraft is now open source!" In the end, based on what they've currently released it appears to be more of an advertisement than anything. With a huge mod community creating projects such as MinecraftForge, most of what is currently released by Mojang/Microsoft is largely irrelevant.

If they release their game engine, however, that's a different story...