Jun,
I am afraid I don't understand exactly what you mean - just a few remarks:
2) A unique name is enforced for children of all kinds of items, not only children of content_folder.
A general rule seems to be to organise page items (those that more or less represent a page) beneath folders, and make items that resemble components of a page direct children of an item.
I wouldn't make page items direct children of other page items, that just doesn't seem to be the intended way. Unfortunately I'm not able to think of any concrete benefits, except from the flexibility you gain when using the folder / index page concept, which allows you for example to move an index page somewhere else or delete it without affecting the URL of it's subpages.
