If one day we have a perfect invisible tech (let's call it magic suit) anyone wearing the magic invisible suit will be fully expected to wear armbands at the very minimum, probably leg bands as well, if not just for safety and ID reasons.
I think the visible armband rule should be mandatory to all players to maintain fairness, if the guy in normal suits have to keep his armband visible (and not allowed to hide it), a ghillie should not enjoy any privilege to be able to remove or relocate or obscure their armband just because they choose to dress in leafs. I would even argue ghillies should have mandatory leg bands in addition to armbands as to allow prone position identification for friendlies.
If I decided to put my armband in my pocket just because I am dressed in some magical optical cloaking tech (therefore should be "fully invisible") they would have kicked me out of the site.
The thing is, this is ultimately a game, and any game needs some sort of balance for A. a fun experience for all and B. for the health of the hobby in general. By and large the real world counters to real snipers are not commonly practised in airsoft or have a much higher bar of entry than just paying £50 for a ghillie overall. This is also a game legally limited to 1J plastic bullets and the rules should be designed around the gameplay that emerge from it and I would say perfect invisibility should be ruled out. At the minimum team identification should be visible from any angle. Outside of the armbands, you can be as invisible as you want but keep the armbands visible.
Moreover if you wear a ghillie you should still need to make the armband (and leg band) visible even just for identification reasons alone. If someone has the rare skills to identify you from under the bush before you can shoot him but not able to tell you are friend or foe, they will be forced to either shoot you or ask you verbally which team you are on, which frankly both are merely workarounds of a problem that should not be allowed to take place in the first place. So this is as much as a class-privilege problem and a practical gameplay rule-set problem.