Assuming yaw is available. Don't count on this.
If we can limit aim to the Z axis and a fixed ironsights we might as well limit steering to pitch and roll.
Jonty's chess analogue is apt: Does it make sense that bishops can only move in diagonals? Is it realistic that queens can move as far as they like in both diagonals and orthogonals while pawns can only move one or two steps forward?
It's a completely broken and nebulous analogy, as you've surmised it. To tidy it up, the rules of chess are akin to the laws of mechanics - they're absolute. Opting to constrain your own mobility is akin to eschewing one's queen. And maybe that could work if it were reciprocal. But if my queen can move in 3 planes and you decide you only want yours to use 2... you'd better be some fearsome chess player.
If every piece on the board moved like a queen - chess would certainly be a much easier game to learn, and it would be a more direct, shortest-path-to-the-target type of game design for sure - but most of the depth of chess would be lost.
So help me clarify this; you're equating a set of symbolic constraints or 'rules' of movement to the 'realism' of motions in 3-space, to argue they're inimical to fun?
OK so by that rationale why not give your queen a 'facing direction' and restrict her to moving in that direction only - with each 45° rotation to face diagonally, vertically or horizontally constituting a discrete move in its own right?
Or why not get rid of the queen altogether - either option could be more fun and require more skill.
But this isn't 'realistic' because the laws of chess are what they are, just as the basic laws of mechanics are immutable and complete; we don't choose either, we just make do as best we can within those constraints.
If however you insisted on bringing a spoon to a laser fight, i agree you should be able to...
Similarly, if your cannons had the ability to turn freely in whichever direction irrespective of your ship's orientation or movement vector, battles would become a simple, predictable chore - you acquire the target, wait for it to come into range, shoot. The one with the most powerful weapon and strongest shields wins. Is this your idea of fun?
what about situational awareness, reaction times, aiming skill, etc. - the better prepared player has the advantage... but is that 'unfair'? Did you
choose a fixed mount weapon because it was more powerful, cheaper, smaller, lower power consumption etc., or just because you felt moveable weapons just weren't 'chess'? Regardless of the reason, if it's a choice then it's a strategy, and different strategies have differing priorities and restrictions. Thus you might attack with a knight or a queen - it's your strategy that matters, not the 'fun' or 'fairness' of their comparative mobility.
It's not so much about making sense than about which set of rules leads to the most interesting gameplay. In a multiplayer game the rules must apply to all players equally, so whatever liberties you allow yourself, you must assume your opponents will have as well. Unrestricted targeting in space very quickly leads to inability to evade fire.
If white and black have the same pieces following the same rules, but one opts not to use a given piece, or to restrict its abilities, should the opponent be obliged in any way - let alone forced - to follow suit?
That's not chess. That's not even tiddlywinks!