That fails if the file is stored on tape or a similar archive. Let's say that there is a big file server with tape storage attached to it and the server automatically moves files that were not used in some time to the tape to save on disk space. This is transparent to the user, so he sees all files and if he tries to access one, it either comes from disk or tape (with some delay).
But if the operating system tries to access all files every time the directory is listed just to determine the file type you get a problem. Or what if the user is connected to the file server with a very slow link (dial-up, GPRS).