I've been instructed by several FAA flight instructors that its necessary to log time sufficient to prove compliance with currency and other regs, but is not legally required to log all time. I have never been instructed to the contrary. If the instruction was incorrect, it'd be useful to see a reference.
As I've mentioned in other similar threads, I know any number of pilots who basically stopped logging time when they retired from airline careers and the like. They log now and again to comply with the regs, viewing as a pain in the rear for just piddle dinking around for fun, but do not log every flight as a general practice.