"overplaying" can "splay" out the leathers much more quickly. And this will lead to more overplaying, because you will need to push harder to get a not to sound properly once the leathers are splayed out.
By pushing the stops part way down, you are increasing the "backpressure". This will cause badly splayed leathers to close faster, hence the better response.
You are correct, if the leathers were is better condition, you would get smooth action at lower volumes when not playing so hard.
I was amazed at the improvement it made on one of my accordions after I replaced all of the leathers. I thought the accordion played well, but after I replaced the leathers, it was a breeze! So much more responsive.
The more the leathers are splayed out the more air you loose before they close when you change direction of the bellows. Playing the accordion to its full potential volumn is playing too hard no matter which you say it. So you get what you got, an accordion with the leathers blown out.
I can replace the leathers no problem, very simple task. I wasn't complaining about the leathers just stating a fact that an extremely loud accordion wants to be played loud in certain jam situations.
With other accordions I have I don't have to worry because the notes will bend when I'm playing too loudly and I know when to ease off the bellows