Well you're right that it's not a silver bullet, but it does have some benefit. At least I know no software running within or on top of the OS is reading/stealing my info.
The baseband has its own processor and, to my understanding, pretty much complete hardware control of the phone.
I don't deny that that the benefit is non-zero, but I think saying "it's not a silver bullet" is still overselling it. I think it would be more accurate to describe it as "It's all I can do, and it's better than nothing."