One of the things I enjoyed in FC 5 was driving around, with a companion using the vehicle mounted weapon, taking out the enemy as we travelled. Unless I'm missing something in FC 6, vehicle weapons are only useful for coop players due to the lack of human companions, which is a big disappointment, especially when I'm trying to escape from a particularly fubar situation.
I initially hoped that random npc guerrillas would hop in and use the turret but that doesn't seem to be the case, which makes it rather unfair that the soldadesca can have a vehicle driver and turret operator in the same vehicle. Because of this I very rarely use the vehicles as you're a sitting target when things kick off.
I suppose it's too late now to hope that the game can be patched to make vehicle weapons useful for solo players by allowing friendly npcs to hop in and operate them, but I guess I can hope.