K3 offered the multiplayer-only game for free on the PSN store, with all DLC maps included (and please think about mixing them all in together!) but there is a level cap on the free version. If you want to go higher in levels, you pay $15. From what I could tell in playing multiplayer, tons of people took advantage of this because the place went wild with noobs. It was great!
The community exploded and a year after the release, it's still thriving and making money. I wouldn't doubt it if many turned around and bought the full game to play the single player, as well.
With the full game's presence on the PSN store along with Crysis 1, this seems like a smart and logical step, imo.