If you see one of your team mates driving a Goliath all alone you should better linkgun-heal it than taking the machine gun seat
Usually you wont do much with that weapon so its often better to just try to keep that tank alive (IMHO)
Yeah I agree. Usually I go with the machine gun only when there is a need to attack targets unreachable for the main cannon (footers behind a hill, flyers...), when there is a need to do high dps combined with the main cannon (attacking nodes and levs), or when the fight involves target that are already (or are very likely to soon become) wounded (which is the case with many fight that involves a lot of actors, like on some TripleSlap skirmishes).
Against a Manta, maybe it's usually better to assist the tank on foot and just go with the Avril / SR.
One another thing:
it seems that some people doesn't know that the secondary fire of the Cicada rear gun exists to *deflect incoming Avril rockets*. (*cough cough anik cough cough* )
So, when the main driver have a gunner, he usually expects to not have to worry about them, since the gunner can easily defend the vehicle against them.
With some practice, even a solo pilot can learn to quickly switch between the two seats just to deflect the Avrils, without losing too much speed (the bots actually do this, at least on Godlike). It's tricky, but it's doable.
However, it seems that it cannot deflect air to air missiles from the Raptor, but I'm not sure (can someone please confirm this?)