These are good questions.
I’m once again speaking for @WG0AT without permission. Hopefully I’m representing his views accurately.
He did tell me that his phone is an indispensable piece of equipment on an activation. He wouldn’t think of doing an activation without it. It’s the same for me. I long ago began carrying an extra battery for my phone, just in case it runs low. (Totally worth the extra weight and space in my pack!) Using the phone for this audio logging technique is just making good use of a fundamental tool that he’s going to be bringing on an activation anyway for other important purposes.
As to the recording not being good enough, I too was worried. I had worked a couple very weak stations that were difficult copy. I wondered if I would be able to dig them out in the recording.
To my distinct surprise, those weak stations were easy copy listening to the audio recording. (This doesn’t make sense to me technically. Perhaps it’s due to some kind of psychological phenomenon. E.g. perhaps under pressure, in the moment on the summit it is psychologically harder for me to copy CW. Sort of a stage fright type of thing.) However, even if the receive audio wasn’t good, the recording of my TX side tone was easy copy.
(I note that I was careful to do a couple sample recordings there on the summit, before starting the activation, to make sure the recording levels were good, etc.)
I caution again that I’ve only done this once (because I had to… to rescue what was an almost failed activation). I’ll need a few more successes before I completely embrace it. That said, it does sound attractive to feel comfortable and confident that I can leave my clipboard, paper and pencils out of the pack like Steve does!
So this is my main interest in head copying (and sending ) CW QSOs.