Lots of reasons folks get asked to switch from gunner, but I'm going to assume this is in response to the known "three engi master race" / "gunner oppressed minority" schism (since almost everyone agrees that more than one gunner on a ship is foolhardy for all except the most exotic of builds).
First let me be open as to where I sit here, for most builds with perfect crews I think 3 engineers works better than having a gunner. You will notice the assumption 'for perfect crews', if you have one of these can I borrow it?
Even with that said, I'm going to make a case for captains (especially captains in pick up games) being more willing to run with a gunner. Advantages of having a gunners (when your crew is not made of magic space juices):
1) Encourages the gunner to keep shooting.
Unless we are trying to flee having one person looking for shooting opportunities (and spotting) rather than trying to fix things is a minimum. If I don't have the tools to be useful fixing stuff then I'm less likely to do it. Plus my role makes it clear what I'm supposed to do, gun. You get something of a mental block to fixing things if that clearly isn't your job.
2) Crew ownership of the boat
You want the crew to think of the ship as theirs. They helped pick their roles, the guns, the tactics. I find crews fight better if they are implementing our plan rather than my plan. Because it can be a fun role odds are pretty good one of your crew will want to go gunner. If you let them you might find they fight a little harder for you.
3) Keeping crews together
A five match old crew fights much better than one freshly formed. Once the crew starts to gel performance generally improves, but how do you keep a crew together? Let them do the things they want, and one of them probably wants to use lots of ammo types and shoot stuff, that is, be a gunner. Sure the mystical perfect crew with none of the trappings of human psychology could operate your boat better by going total airship combat and rotating out the roles like some crazy Dutch sky ballet, but is that something you are going to get from that level 6 gunner who just joined the ship?
Now with that said I think there are lots of cases when a new player should be willing to switch from gunner:
1) Ship already has a gunner.
Unless I'm doing something crazy I don't need two gunners. I'm happy for you to stay as an engineer, and I don't mind if you go to another ship, but those are the two options.
2) Load-out will be seriously suboptimal with a gunner
Some have said the only builds suboptimal with a gunner are the metamidion and the meta-junker, I don't buy this. On Crazy King my squid doesn't need a gunner because killing things aint my job. This isn't super common (most ships will function just fine with a gunner or with a third engineer and the human element messes up the min-maxing calculus the two religious factions here engage in, metamidion being a good example; if you have a good gunner a gunner can be better, a good engineer and an engineer can be better), but if you look at a ship and cant think of a decent reason for a second ammo / third ammo type then you want to switch, or find another ship.
3) This match will end poorly if I have to train you
Some guns are hard to shoot. If this is game number 4 for you or your answer to 'have you fired a lumberjack before?' is 'what's a lumberjack?' I'm probably going to either swap load outs or get a more experienced crew member to gun unless our opposition is weak. I like to train newcomers but dumping a load of pressure on you with a pivotal role when you are inexperienced is unfair and unfun. This is only likely to happen if I know my crew is invested in the load out I've picked since I usually gear the load out to the crew, not vice versa. If that isn't the case I will leave you as gunner if that is what you want and find you a more gentle introduction to the art of making metal sky confetti.
Unless training for a competitive game most of the time the solution to this problem, to the extent that there is a problem here, is for folks to be a bit flexible and try to find a ship configuration that makes everyone at least somewhat happy.
