If agility were merely an ability to "spin", it would figure into the "Batteries Bearing" equation IF GDW also wanted to further burden the rules the were writing.
Regardless of spin rate, or lack of it, the same % of batteries would be bearing over time. But that aside, your main point is that its an abstraction, a 'design for effect' rule.
Military pilots speak of "energy states" when discussing dogfighting and other combat manoeuvres. Those energy states involve the amount of thrust the engines provide but they also include things like g-stresses, abilities to change facings, and lots of other stuff.
Energy state is more typically thrust, momentum, height and air friction (from memory & fuel state fits in there somewhere). Giving the pilot choices at any particular moment on how to use or increase accumulated energy. Energy is bled off or gained through choices. Light the afterburners & gain speed and/or height, costing endurance. Diving releases stored energy giving speed, costing height Turns add to air friction and cost speed & height unless thrust is added to compensate costing endurance instead.
The same equations are simpler in space combat, endurance, height and friction are not significant factors and the MD's can and likely are on max output most (if not all) of the time. Turning doesn't automatically slow momentum. Instead turning within the front arc will continue to increase momentum. Whilst turning within the back arc will decrease momentum. A pattern of violent evasive manoeuvre from the En perspective, can be performed at max thrust and still generate significant thrust toward or away from the En. Especially at the closing/parallel vectors assumed of HG combats (vs the high speed pass).
In a nut shell, I'm not sure energy state is a good analogy either.
So we are left with the observation that agility uses the MD.
- MD's in Traveller thrust in one direction only (give or take a little)
- Agility uses a lot of EP's to enable use of the MD, involving ship attitude change to enable MD thrust in new directions.
- EP's are scaled to match ship mass. The larger the ship, the more EP's are needed to generate the same attitude change rate.
- Manoeuvre jets can be assumed in all vessels for docking manoeuvres and attitude change. Faster changes require bigger jets / more energy.
- Any change in attitude will result in g forces at the extremities of the vessel. Faster changes result in more extreme g's.
The main driver is to explain the high cost in EP's, something has to be using them. My logic is that manoeuvre jets and lateral grav compensators are the short term high energy use components. Taking high energy to change attitude fast (& stop the high speed spin fast) and enable the lateral grav compensators to adjust to maintain a safe working environment.
IMHO its the HG time scale that nullifies most of this. If I'm correct, and I accept its unlikely anyone will agree with me (been here before...
) any attitude changes, particular fast ones enabling near instant changes in thrust direction is well under the 20 minute time scale.
Even in vector or hex combat, a time scale of perhaps a minute or two would be needed to accurately reflect fast attitude changes. And space combat at that time scale doesn't push my buttons. But although my math isn't up to it, it would be interesting to see how fast you can spin various length ships without exceeding 6g at the extremes. If the game reflected it, it might have made the sphere (Tigress) the shape of choice for very large vessels.
Those agility ratings are not prefect but they work well enough. More importantly, they work fast enough and they trigger design choices just as real world concerns about aircraft energy states do.
Agility is a game artifact. Trying to understand it at a deeper level than that is fruitless.
Ahhh, your a stick in the mud Bill.

We know there would have been an underlying logic & its interesting to speculate. Having said that, I agree that a deeper understanding of agility is not a requirement for playing the game and agility does prompt design choices that 'feel' appropriate. So whilst I agree completely with your sentiment, I still haven't tired of the discussion.
Indeed this thread has made me adapt my 'grav compensators' theory to also include considering the significant energy required for the jets to spin a large mass at speed and then stop the high speed spin. The more efficient the spin, the more efficient the use of the MD & the bigger the circle of possible positions the target ship may be occupying.