I think that maybe the mindset you're thinking about stems both from the need, whether perceived or real, to have someone else who is capable of minding the helm in the event of primary operator incapacitation...because they can.
If your car had dual controls - that is, if the passenger seat had steering wheel, brake, accelerator, etc., making it possible for someone else to take over, wouldn't you be more comfortable if the person sitting there knew how to drive, or at least how to safely get the vehicle stopped in the event that you (God forbid) became incapacitated?
I know that if such were the case with my car, all of a sudden I'd feel differently about who sat in the passenger seat. And, I think that the very reason I don't worry about it now is because it's simply not an option.
Seems foolish not to use an option that's available for the safety of the vessel and its passengers - like making sure my wife can operate the boat to some degree. Tempting fate, maybe.
If setting out for a long offshore passage, would you take an EPIRB? GPS, and backup GPS? Liferaft? None of these things are necessary, strictly speaking, but again...seems foolish not to utilize readily available safety measures.
Just my $0.02.
And, my apologies if my thoughts aren't flowing very cogently this morning...first cup of coffee hasn't kicked in yet.