Pulse1,
I agree with you. Most light aircraft are certified for single pilot operation and as such can only have one pilot. i.e no co-pilot as in most airline operations. My point about the tech log is simply that the commander of the a/c signs the acceptance of the a/c and therefore remains responsible for the a/c at all times.
On a single pilot certified a/c, if you have 2 pilots on board and one is always responsible you cannot hand over to the other pilot since you would then have 2 pilots in a single pilot a/c.
Take this as an example for legal purposes:
2 pilots flying in a PA-28 break the low flying rule which is reported by someone on the ground. You don't find out for a few days having signed the tech log (whether it be before or after flight). However, at the time the other pilot is flying the a/c since you handed control to him, he then denies being in control. Legally who is responsible and who carries the can?