if you're relying on such feature, you'll probably serve less time being charged with destruction of evidence...

If the Duress PIN is an obvious one, it may be one of the first ones your adversaries try. Like 1111 for example. So you may not even have to tell them the Duress PIN for them to attempt it.

Surely that's better than being charged with whatever crime they're trying to pin on you?

It depends, often the cover up is worse than the crime. See: Enron, Watergate, Trevor Jacob

They'll have to prove it.

How would they know? Genuine question, I don't run GOS.