I see those pins now, I wasn't looking at the external connections (traces) but the pin names in each device.

You hit the nail on the head, leave the card in and have the ability to trigger whenever, including the software detectable ones like you suggest.

There's less likely chance that you will damage a button, or lose it, by pressing it to start/stop a datalog there there is when you insert/remove an SD card. ;)

I always found it annoying when I had to perform similar operations with other devices, such as (and this will date me a little) cassette players in cars, for many years you had to insert the cassette to listen to it, which is fine, but then to go back to radio, you had to eject the cassette, instead of just hitting a source button. ;)

With the source code being open, that does change things a little. :)