this has nothing to do with seatbelts, bad analogy because seatbelts do not impact the safety of your driving or the way you drive, they impact your safety when you crash. BCD's are a tool that is used to make diving easier, and possible under certain circumstances.
To reiterate. BCD's are buoyancy compensation devices. What needs to be compensated for? It accounts for the loss of negative buoyancy by the gas volume that is exhaled during the course of the dive, which in a cave dive can exceed 40lbs. It also compensates for the loss of positive buoyancy attributed by compression at depth of closed cell, flexible wall vessels *i.e. neoprene*. When the combined extremes exceed the comfortable capacity of your lugs, which is dependent on the specific individual, you need that tool to compensate.
In the early days of cave diving this was done by cutting the neck of a clorox bottle off and holding it in front of you and exhaling into it basically acting as a lift bag. Now it is done with a myriad of different BCD designs.
Now, surface safety/comfort is a different thing altogether and something the equipment manufacturers are very clear about putting on their bcd's. They are NOT personal flotation devices, we use them like PFD's, but they are not.
If you are diving in a bathing suit with an AL80, in clear sunny florida, do you need a BCD? For normal sized people, no way because the bcd doesn't need to compensate for a whole lot and as long as you are weighted properly, it's a non-issue. For the examples brought up above where you can't hover because you're too negative, well that's for you to decide if that is a tool that you need. I personally won't dive in anything more than a shorty without a BCD, but in swim trunks I have no problem diving with an AL80 in fresh or LP72 in salt with no bcd