Then why GPL at all? I'm sure the community has disassemblers, debuggers, emulators and logic analyzers to dissect the interface.This is not the case. Companies can afford special equipment because they have capital gains to reap from using it properly. The general public most certainly does not have the financial basis for access to this sort of equipment.
This is what necessitated the GPL in the first place -- too much proprietary systems and code, and not enough people and tools to sit around and reverse it all day.
The company's competitors will probably rip off their code and designs either way (whether it's open or closed), as overseas cloners have entire divisions dedicated to mapping silicon at the gate level and documenting disassembled code line by line.
However, if it's GPL, they still have a legal basis to go after those competitors, while providing the public with obvious benefits, and thus a real reason to buy their product over a competitor's.
A GPL software interface holds far more weight for me than commercial propaganda ever will, in terms of my desire to purchase a hardware product. I am certainly not alone.