Hi Oleh,
It’s true that QRS CW is easier to be copied when the signal is very weak, and I often call QRS (18 WPM) to DX activators with good results. Fast CW when signals are very weak is very hard to copy because any little QRM or minimum QRN crash can smash almost the full character sent (letter, number, punctuation…)
QRS code is also easier to be copied by the activator in the middle of a pile-up.
However, I personally don’t feel comfortable with activating QRS below 18 WPM, as this extends the time for the activator to work all the pile up and this is not often possible due to time and weather constrains.
With my usual activating speed of 22 WPM and my fair farnsworth practice whenever necessary, I never ever found a problem of a chaser not able to chase me due to being unable to copy my code or, at least, I’ve never been made aware of such situation.
I think activating at 10 or 15WPM in order to give a chance to one potential CW beginner one day, doesn’t worth it because it penalises on the other hand the whole group of chasers all the time everytimes. This also punishes the activator having to spend more than double time in the summit to complete the activation. That’s not fair from my point of view.
Best 73,
Guru