Quoting bwilson@clickshift.com, from the post of Wed, 08 Sep:
Using subdirectory trees fixes an ancient UNIX problem that happens when there are several hundred of files in one directory -- if this is allowed to happen the system will sllloooooowwwww ddooooooowwwwwnnnn every time the full directory is accessed.
yo! don't generalize! my ReiserFS deals quite fine with tens of thousands of files in a directories, and probably more :-)