Isn't APB like 32 people or less per server? And you only interact with a couple at a time when happen to get missions in the same area? Yea, sounds like a $50-60 free to play game that they wanted to charge MMO fees for. I wouldn't pay monthly to play Modern Warfare, so why would I pay monthly for this? If I'm paying monthly for one game, it needs to be a hell of a lot grander in scale. For a comparison in players per server: WoW has 10,000+ players on LOW POPULATION servers. The high end ones are 20,000+. That's being part of a large world, not dropping in for some capture the flag or deathmatch real quick. That's the sort of environment that justifies a monthly fee to me.
EDIT: Correcting my numbers on WoW populations. Apparently some servers have around 500 players, but that would be like a ghost town. (who was it that said they'd rather play WoW as a single player game? There you go, you'll almost never run into anyone. Though there's still monthly fees.) The most populated server has 33,000 players. (20,000 alliance and 13,000 horde)