I hear what your saying A and C, my panel mounted 496 has an internal battery that's good for at least a few hours, it detects aircraft power and switches accordingly which allows me plenty time pre engine start to get the route all set up. If I was in a rental with the meter running I probably won't bother.
Regarding the KLN, when I was taking my instrument checkride earlier this year I was required to do one GPS approach, strangely this was only the second time throughout my entire IFR training that I would be using the KLN for an approach, or at all for that matter
I sat down and read the manual cover to cover a few times and somehow managed to load the approach for the checkride. After passing, I had a few fun vfr flights where I tried to use it again and just could not get the hang of it.