Well, the gun on the big Mac is its own separate ship, so basically it's a gun pod, and apparently the only way to operate any gun pod in macross is through the use of fingers.
as for the transformation, I've always liked to believe that they retained the transformation because NUNS tactical doctrine dictates that if using the main cannon isn't an option and other weapons systems are ineffective, then the macross is to transform to storm attacker mode, move in and beat the crap out of the enemies’ capital ships
(Admittedly there's absolutely no reason to believe this, but it would be pretty awesome to see the Big Mac Falcon Punch a Vajra Carrier. )