In A2 objects attached can be seen some times but they're usually drawn in the wrong position or wrong scale, I assume it's part of the same issue in A3 although I haven't looked at it myself.

Actually, this is due to the way Arma2 and 3 render the scene in multiple passes (foreground / background). Essentially it's a z-sorting issue, and it's the same in both games.

The player's body/vehicle (including all vehicle occupants) are the "foreground" and are always rendered in front of everything else (the "background"), which is why you never see your weapon or vehicle clipping into any environment objects from 1st person view (even if you do see it in 3rd).

Since objects attached to a vehicle are still considered to be in the "background", they are always rendered behind the vehicle, even if they are physically inside it. This is what causes the supposed position/scale issues that F2k_Sel mentioned, which is in fact just an optical illusion.

Anyway, BIS would have to move the player's vehicle from the virtual foreground into the background in order to solve this issue, which could cause a couple of new issues (mostly visual, like clipping of grass through the vehicle floor).

I guess I'd rather not have the clipping and leave things as they are. A shame though - be brill if BI could give us the best of both worlds!

Anyway, BIS would have to move the player's vehicle from the virtual foreground into the background in order to solve this issue, which could cause a couple of new issues (mostly visual, like clipping of grass through the vehicle floor).

Wouldn't a much simpler solution, if possible, be to consider units "attachTo"-ed to the vehicle in the "foreground" for the purposes of first-person rendering?

It would still retain the benefits you mentioned, but would solve the issues of attached units being rendered oddly.