I haven't taken extra armor on a vehicle since the old marine codex, and the more I think of why, it just keeps coming back to the same thing: Shaken is almost just as bad as stunned, the only real difference being for transports with a YOU MUST GET THERE NOW type of payload. But that honestly rarely comes up in my games, if at all; most of my troops are where they need to be when their tank finally goes down, or close enough to get there on foot reliably. I've never had a situation come up that made me go "Aw, if only I had taken extra armor!" Have you?