one of these being placed in teh MAIN base = two extra vehicles (with 50cal goodness) for very little cost.A maximum of four concurrent Forward Outposts (either Bunkers or Firebases) can be deployed for your team at any time, no matter the number of CPs.
And best of all, the CP is right there for you to reload the truck's supply boxes and go build another one almost straight away.
So, be smart and get a bunker down right at the start of the game.
This isn't a good idea for all maps, e.g. Mestia benefits from FB's placed in favourable defensive locations in the capping areas.