With some laptops if a phone isn't charged enough, a USB connection to it won't stay stable if the phone is trying to charge at high levels. This could be an issue for using a hotspot before Airplane-mode-time or syncing/backing up if you do that across the wire. Results will depend on the laptop, it's age and the type of USB port.

When charging is available onboard, people who plug in will hit 100%.

Last check-in with AA they were fairly clear "No laptops, phones or anything like that in the checked bag." They seemed more concerned about that than if I had packed my own bag and had it been in my possession at all times.

10 years ago, they were freaked about evening having cell phones onboard and now they don't care if you use them at any time from 0 ft to 30K to 0 ft.

More than a couple of companies sell bags to contain a fire if it starts and their websites seem to imply they airlines are buying them.