Lots of great input so far, thanks everyone! I thought I'd add some comments, clarifications and thoughts of my own.
First of all, yes Battledress is expensive. So are G/Carriers, Grav tanks, APCs and other support vehicles that infantry are traditionally dependent upon. Battledress-equipped troops also benefit from all of those, but don't need them in order to be effective.
Battledress wearers can fly (thanks to grav assist, which can also be fitted to combat armour to be fair), are practically immune to regular infantry weapons, and they can carry destructive and/or traditionally vehicle-mounted weapons such as P/FGMP, autocannons, VRF Gauss guns, tac missiles...
There are drawbacks, of course. The suit is expensive, complex and requires lots of training (vacc suit, flyer(personal) heavy weapons) and the fuel cells only last 12 hours. Duration and life support can be extended, but only by occupying slots that could have mounted weapons, support equipment or grav harness...
The Azhanti class frontier cruiser carries a company of 150 men. Equipping them with battledress makes them so much more effective, especially without support vehicles. A dedicated troop carrier should have space for vehicles, but a frontier cruiser only has 400 something dtons of cargo, and they need supplies and spares for the ship too.
I’m not sure five regular soldiers versus 10 battledress is a fair comparison. Take a ten man squad, wearing combat armour and using gauss rifles. Their opposition: one battledress-equipped soldier, armed with a VRF Gauss gun with HEAP ammo. The lone BD trooper could almost wipe out the entire squad within a single combat round! I’m reminded of an old 40k quote, goes something like “give me a hundred space marines, or Failing that, a thousand other troops”.
So basically, I’m more and more in the “can they afford not to?” camp.
Lastly, I love the idea of giving naval gunners and pilots scout battledress! The other day I thought about what fighter pilots might wear, and in my modest small-minded world I assumed vacc suit rather than combat armour... but scout BD would make a lot of sense!
There was a fighter discussion not too long ago here, and it was suggested to give pilots skill augments and similar upgrades. A scout BD would give similar bonus, but when the pilot retires or dies the suit can be worn by their successor.
A pilot in battledress is also more survivable, plus they could most likely fly back to their carrier using builtin thruster pack!
Final, final thought: equipping not only marines but also gunners (and possibly other crew too?) with battledress makes boarding an imperial navy vessel a very difficult task...