Disconnect battery when parked?

I don't think that is 100% true. If what you are saying then how would one turn on the four way flashers, lock the car on the side of the road and proceed to get help if say you ran out of gas ?? The interior light, horn, cigarette lighter, headlight and possibly a few other accessories will operate without the key in the ignition. So to answer the OP's question, it's not a bad practice to have a quick disconnect switch and to use it. Just my thoughts.
Marker lights and 4 way flashers for emergency use bypass the key, as do the head lights and interior light in most cases. As long as their switches are in good condition and turned off, no current flows. There has been some recent recalls due to poor build quality ignition or headlight switches that transfered minute amounts of contact metal that leaves a small current path. Eventually this builds up to the point enough current to get hot transfers. Bean counters cutting corners is the cause. Of course the bean counters finger engineering instead of accepting blame.
Dad had a street rod with a SBC engine in it. He installed a battery disconnect as the battery would be dead in a day or two if he forgot to turn it off. I checked it and the pprevious owner had wired the battery and key power terminals on the Delco alternator backwards. Battery power on the key power terminal leaves the voltage regulator on all the time and drains the battery. I simply popped the the wires out of the terminal connector aand reversed locations. Half hour or a bit less time spend. Dad thought I am a total genius as his battery stayed charged. He would shut the disconnect off in the fall so the radio clock would not drain the battery over the winter.