Show me a motorist who flashes their lights, to let someone out , (in an attmept to get a warm glowy feeling for themselves) and I will show you one who hasn't looked in their mirrors. Either there is no one behind them, and everyone else would be better served by them just moving, or there there are X people behind them and they have now held X people up for the sake of one other person.
I am afraid I also have a small issue with your blanket logic.
There is a busy road outside Oxford that is a pig to get across and on to from side turns. Some may have heard of it; the A40 - the particular point that springs to mind is by the Evenlode pub.
Constant traffice in Eastbound carriage (single lane / file at this point). There are always cars queuing to cross the East bound lane but they sit in effectively a filter lane as long as there are only a few of them.
So - I check my mirrors. If a gap behind me, don't worry they can cross after me. If piles of traffic as is the norm, and nobody has shown COMMON COURTESY ahead of me, then I will often flash my headlights a few times to show I am there, aware and noticed them
(We all know what I mean)
Once I have slowed slightly and allowed the gap to form in front of me, alert drivers who are paying attention will 'nip' across my front in the gap. Numpties wont and I wont flash them again or stop! Within a few seconds, the gap will naturally disappear in front of me as I rejoing the constantly moving queue.
I sometimes think it is a shame that COURTESY and 'Be Nice' are not the first rules of the road - anyone else read 'The Second Coming' (John Niven) recently?