It seems odd that they would release it like this instead of just delaying it until the rest of the content is ready. Half the maps and most of the modes not being available at launch seems very different to me than simply a couple maps added here and there.
The game ships with half of the maps and half of the modes eventually promised by August. Technically, had they said nothing, the game is launching with five maps and two modes which are 100% of the content. They're intent is to assure future support and growth as the game (if you discount the single-player) is not that big.
Obviously, they want the game to be out and selling before CoD et al hit the market and bury everything under foot.