Illumians don't make sense.
A secretive, infomation loving, superspecialized people with glowing runes around their heads that nobody knows about?  In a world with divination spells and guilds of wizards that specialize in them?  In a  game system with a telepath psion path?  Bardic knowledge? loredelvers? crystal balls?  Heck, a gm can barely keep the lich's pin number a secret from a high level party.
Also, as a gm, you can't retroactively include them in a setting with out rewriting everything.  They just aren't "add on" friendly.
The elans, on the other hand, are a small group of highly selective, slowly recruiting, normal looking...people.  You can add them in with only as much trouble as adding any other obscure "cult".