But there are critical differences: most importantly, the article namespace remains semantically flat; "Foo (Bar)" is still a top- level article just like "Bar", and they have no pre-defined relationship of any kind, and no runtime features that link them in any way. People won't be tempted to use them to "categorize" pages into hierachies.
That's true. With subpages the new user saw a link called "/blah" and in the edit page it also said [[/blah]], which makes it very easy and tempting.
But an automatic replacement of [[/bar]] to [[foo/bar]] on Save wouldn't be any different, except the () soultion might be more "natural" looking than the / thing...
Magnus