Missed one (which I still consider valid): making it trivial for others to extend the firmware might endanger other products on sale. ATI learned this a few years ago, when it was discovered a software upgrade was all required to turn a sub-$150 graphics card into a far more expensive model.
From the user's perspective, all they see is hardware that can Do So Much More simply with a little firmware jiggery, but from the manufacturer's perspective, they've spent billions developing that firmware and if they choose to differentiate depending on its configuration, and they've paid the infrastructure costs to reach this position of privilege, then as far as I'm concerned that's their prerogative.
From the user's perspective, all they see is hardware that can Do So Much More simply with a little firmware jiggery, but from the manufacturer's perspective, they've spent billions developing that firmware and if they choose to differentiate depending on its configuration, and they've paid the infrastructure costs to reach this position of privilege, then as far as I'm concerned that's their prerogative.