Someone was lamenting the underuse of the Dark Creeper and Dark Stalker from the Fiend Folio. I do kind of like them, too, although maybe their tendency to explode in a brilliant blast seems out of place. I think their underuse is due to them fitting into the "small underground humanoids" niche that was already well-stocked and became overstocked with the publication of the Fiend Folio.
The answer, of course, is that you don't use these guys in addition to goblins, kobolds, xvarts, and so on, but instead of. They are the alternative subterranean race for a different kind of campaign feel. I think I'd like to use them with my Forge Rats and Sage Spiders (from the MegaLex project.) An underground settlement of magic thieves, sentient rat-smiths, and scroll collectors make an interesting source for magic items.