The Elemental Princes of Evil, despite their lackluster illustrations, always impressed me as an interesting variant on the typical evil Ds (demons, devils, daemons, demodands...). Combined with mephits, they provide a backbone for elemental evil... which filled me with some imagined threats back in the days when the Temple of Elemental Evil was just a name mentioned in the Village of Hommlett module and not an actual published module.
The idea of cults worshipping the destructive powers of elements, perhaps as foils to the cults of Druids, and with modified clerical powers channeling elements seems laden with real possibilities. As actual monsters, the Elemental Princes carried with them the same problems as the Demon Princes and Greater Devils, but as far as populating those evil cults, pure gold.
The Mephits also seemed to fill in the Imp spot for the evil priests and wizards, bringing some twisted nastiness into the equation and giving a basic opponent for low-level characters. Bringing in efreeti, yeti, and whatnot into the mid-level opponents gives a whole hierarchy of elemental evil, and the possible backbone of a campaign.