One also has to keep in mind that if user2 moves the directory to a new disk, user1 can not do anything in this directory.

Read also about this (from wikipedia):

Quote:A file's inode number stays the same when it is moved to another directory on the same device, or when the disk is defragmented which may change its physical location. This also implies that completely conforming inode behavior is impossible to implement with many non-Unix file systems, such as FAT and its descendants, which don't have a way of storing this invariance when both a file's directory entry and its data are moved around.

Now if we have a FAT fs mounted and someone was to rename it, it might cause other links to break and/or the user currently being in it to not be able to access it anymore.