The important thing to remember about FOBs, though, is that they're open to invasion from other players. Of course, that means other players' FOBs are open to invasion as well, and if you're sneaky, you can easily fill your coffers with ill-gotten gains. Only resources procured online are susceptible to theft, so you don't have to worry about losing stuff you earn from the single-player game. FOBs aren't necessary to complete the game, but I find the bonuses they provide to be worth the hassle of maintaining them.