What if we change the default icons? Then the skin will look different as intented.
Exactly. If the skin creator specifies no icon, it means he's using the default one for that item. That's the whole point of allowing icons to NOT exist. No item = default icon. If he wants it consistent, he must add his own icons, but if he doesn't, we use the default.
If I was developing a skin that only changed the toolbar, I wouldn't want the rest of the GUI to be stale with old icons from old versions of aMule because I had to include copies on my .zip file. I want them to be the default ones, automatically used by aMule.
By including these small icons we ensure that the skin will look the same in aMule 2.4.0 as it will look in 2.2.0, no matter how the default icons look like.
We actually ensure that 2.4.0 will look as 2.2.0, no matter how we change them upstream. Or force the skin creator to update his skin with the 2.4.0 icons, and look different on 2.2.0. It's a bad idea.