Without exaggeration, I'd say about 5% of the cases?
> • you memorized it
This means I've got about a day or two at most while I still have it memorized - but my plans usually span longer than that.
> • it's not as important as it seemed yesterday
That would account for most of the irrelevant 5% of my notes, I suppose; but most of the time, it will still provide important context when I come back to it weeks later.
>• it's a todo and having it there actually nags you and prevents you from completing it
Never really understood these sorts of todo issues. If a task is sitting somewhere for longer than necessary, it's likely unimportant. If it is important, I'm more than glad to be "nagged".
>• it creates some other kind of mental load: while skipping over it, it grabs your attention, which then prevents you from attending to other tasks on the list.
Move it to the bottom, or to another list outright ("To do later").
>I have a theory that GTD helps only few, for most people it just makes them feel better because they are now spending time creating lists instead of doing what's on them.
I have a theory that people like to riff on organizational tools when they do not know how to utilize them properly - which is why most are drawn to utilize them in ineffective and inefficient ways, generalizing this as a property of the tool.
Besides, GTD/task management is a miniscule portion of why it makes sense to construct knowledge bases.