I would under normal circumstances, but if you're alone/in a place you don't know/drunk/woman/at 2am, or even all of those, that's a tough decision on whether it's safer to get in or stay out, and we're socially conditioned to not question these sorts of situations as much as we probably should.
Unless you called a cab to escape a particularly dangerous situation, I can't imagine it ever being a good idea getting into a car with a different driver and registration. You're not getting into a taxi, you're getting into a stranger's car.
