I view this really as a trick for the more obtuse talks. I can fully take and benefit from notes on something i understand. But if i end up in a talk that is going to be beyond me, having the goal of not taking notes, but picking tidbits is often the only way to get anything out of it. If i take full notes i forget to listen as there's simply far too much to write down. If i take no notes, then after the next couple talks I've totally forgotten everything i "learned". If i take very few, short notes then i can still pay enough attention to try understanding but i also have a couple anchor points from which to remember the talk.
It's just a balancing act, especially when you're in a conference where you have to pick a new talk every 30 minutes for six hours a day for 4 days. It can get pretty ridiculous and having a better strategy than "just remember it" or "well write everything down" can be very useful when the most interesting talk to you is in the middle on day 3.
This does sound like a psychological crutch 'living in the moment' technique of feeling good that you've achieved something (learning 3 things) rather than understanding the material or atleast the nature of the area/problem described in the talk.
Would the same benefits be had from skipping the talk and reading the conclusions slide?