Mind you, OS X makes extensive use of extended attributes in addition to resource forks (and it's largely deprecated resource forks in favor of app folders). Spend some time poking around Siracusa's reviews (since Tiger); he loves to go into detail about every new way Apple makes use of extended attributes.

Also, it's not fair to say that almost nobody uses them. Chrome makes use of extended attributes, as does KDE's metadata system and a few other things.

> (limiting their size to strings that will fit in RAM)

That's an understatement. The Linux kernel API limits the size of all extended attributes to 64KB, and the most popular filesystems limit them further to 4KB. That's not really comparable to a true fork.

ZFS is the exception: its extended attributes are implemented as forks, and the maximum size of an extended attribute is the same as that of a file. Unfortunately, those aren't accessible on ZOL because the kernel won't support it, so you can really only take advantage of it on Solaris/Illumos (and maybe FreeBSD?).