Come on, you know REC can't be used for buying items, it's rental credit and atm the store items can only be bought with non-virtual currency. It serves as both an almost neverending fundraiser for the game and capitalizing on SC whales.
Sure, you rent them for a week. But a single game of Vanduul Swarm nets you almost 20,000REC if I remember correctly (I think I got over 18,000 for clearing all 12 waves a while ago), enough to rent any weapon for a couple of weeks. If you play, you constantly get more REC. If you don't play, what would you want missiles for? Not to mention that the most expensive class IX missiles are completely useless right now, because there are no capital ships to shoot.
Also, and that is a real problem, nobody knows what "buying" missiles actually means. You won't get a free lifetime supply in the final game. You'll get a certain number of missiles, but nobody knows how many. That's why even CIG's community managers suggested not to buy anything other than decorations and skins on Voyager Direct. That store is a fucking mess right now in general. The option to melt down Voyager Direct purchases has been announced more than a year ago and is still not implemented. People suggested removing limited use weapons from Voyager Direct, but CIG didn't do that, either.
What if you bought those missiles, fired them, and
they missed their target
The same thing that happens in EVE: You wasted money. But you don't buy/ rent individual missiles in the alpha, you get an unlimited supply.