Hey Kirk,
I only call them "aliases" just because when I make a traditional UNIX symlink (eg "ln -s (path/to/source file) (path/to/linkname)", when I look at it in the Finder it refers to it as an "alias".
Basically I have a script which scans the EXIF contents of several thousand photos across several directories on several drives on several machines (whew), and based on certain conditions, it will populate a single directory off my Desktop with symlinks. This way I only ever have to work with the contents of one single directory, rather than spending all my time navigating through various folders in a GUI to manipulate the the photos I need.
That is a good point about the attached/sidecar files though and am not sure the answer on that one. I would think that PM would just follow the symlink if available to a given file. It isn't PM's responsibility at this point to make sure that the sidecar files also have corresponding symlinks...that is on the user. If new sidecar files were created, then they would go to the same location that the symlink resides, not to the same destination as the symlink. Again, the user's problem to deal with.
This is how UNIX works anyways. At the command line you make a copy of a file which happens to be a symlink to a file elsewhere, the copied file is stored in the current working directory, not at the symlink destination.
Lastly, PM could get confused if you browsed two folders, one containing the original files, and the other containing symlinks to the files. They're both actually the same file, but unless PM goes out of its way it won't know that they're the same file. This could lead to problems where you've edited IPTC Info in one tab and when you switch to the other tab and view IPTC Info you won't necessarily see the same data.
It shouldn't matter should it? PM only lets you edit one file at a time in the IPTC window. Doesn't PM refresh the directories? (eg, if you changed the date/time of a photo, doesn't PM refresh to show this change?) I'm not up on the internals of PM, but if the mtime of a file changes in a tab, doesn't PM refresh that file in the tab?