(Avast ye spoilers)
I tend to agree. Nanny really didn't accomplish all the much in CJ, but, then again, what does she really do in any of the witches' books that make a real difference? In Lords and Ladies she'll ill-advisedly brings the villages to the Stone Circle in the misguided notion that they can defeat the Elves, and is proven almost fatally wrong. Sure, she convinces the Elf King to leave the sweat lodge, but to me this was totally unsatisfactory, because his deus-ex-machina appearance kept Magrat from possibly killing the Elf Queen (and, by extension, prevented the villagers from being able to destroy all the Elves), leaving that scenario as a glorified stalemate that required Tiffany Aching to ultimately resolve--twice. She spends way too much of her time pursuing her faus-romance with Casanunda, ignoring her duties as "watcher of the edges" while Granny Weatherwax, as always, has to do the real work.
What major accomplsihments does Nanny achieve in Witches Abroad, either? She loses all of their money in a crooked card game. She really doesn't do much of anything of huge significance in resolving the battle of the Weatherwax sisters.
Other than making sure Granny Weatherwax always gets her sweets or spearheading "side projects" (such as engaging the Nac Mac Feegle) that really have that much of an effect on the main story, Nanny is, at best, a sidekick character.
Sure, she is a lovable character, but CJ makes it quite evident that she's totally unworthy of fulfilling the "crone" role of the witch triad when Granny temporarily retires from the game.