For me, a major reason to carry a semi-auto with one in the chamber is that racking the slide to load the first round is one of the likeliest situations to have a malfunction. You might be able to draw and rack the slide fairly fast, but if you end up having to rack, mis-fire, tap, rack you’re in for a lot of trouble. With one in the chamber you’re almost guaranteed to get that one round off. It also the only reason I cary a spare mag. It’s not that I think I will need the extra rounds as much as a mag failure is the most common failure with a semi-auto so being able to drop the mag and insert a fresh one is invaluable.
As far as having to unload to store the gun in your vehicle when you’re going into a prohibited location. I’ve decided that, once my vehicle is engine off, in park, emergency brake on I am no longer transporting my gun, I’m storing it. You don’t need to unload it to store it. I like to minimize the amount of handling of my carry gun. It’s always loaded, one in the chamber and in its holster. At night, I take it off my belt (in the holster) and stick it in my bedside safe. All my vehicles have Console Vault safes in them and when I need to disarm I again just take the gun, holster and all, and put it in the safe. Other than when I’m going to clean them, the only way my carry guns are unloaded is by firing off all the rounds at the range.