If someone has evidence on their encrypted drive that could put them away for life or a large portion thereof, or even trigger a capital case, the fine or few years of time behind bars for whatever piddly-ass crap the prosecutor can dig up is not going to be their primary concern. Worse for the prosecutor, if they go to extreme lengths to find something to charge you with, they risk pissing off the judge and/or appellate system with their vindictiveness.

You're always going to have to make a judgement as to whether you're better off cutting a deal or refusing to cooperate, but that does absolutely nothing to change the fact that a hidden volume may save your life.