There's no reason for them to reassemble all of the witches at the same time. It would still be stupid even if the room was full of guards.
There actually is a reason, although it's one of convenience more than absolute necessity. To keep vampires alive while normally maintaining them in fragments, they need to be fed pretty often, and on a tight schedule if they want any safety margin. They can't just be kept in small immobile chunks indefinitely. It's quite a production to handle the healing-and-feeding phase (Alec or Alec!Adelaide needs to be available, you need to fill up whoever feeds them so their food doesn't get stolen by inadequately fed guards, you need to have extra food on hand for the witches themselves, etc.). Heidi has to make a bunch of trips for extra food. She has to range farther than usual to get it so there won't be too many concentrated disappearances all at once in the near environs of Volterra. This all takes a while. Someone else could take on some of the hunting duties, but except for Heidi!Adelaide, nobody is quite as well-equipped to unobtrusively bring in an entire delicious tour group all at the same time, and Adelaide usually has other people to copy and other tasks to accomplish. Every extra guard they put in the room is somebody they have to saturate with even more blood so they can handle being there.
The best candidate for the idiot ball action here is that they didn't time the capturing of Jacob's pack for when the witches had already just been fed and were in harmless bits and pieces.
you need to fill up whoever feeds them so their food doesn't get stolen by inadequately fed guards
Couldn't they just get one of the imprints to do it?
This is Part 2 of the discussion of Alicorn's Twilight fanfic Luminosity.
LATE BREAKING EDIT: Part 3 exists now, so new comment threads should be started there rather than here.
In the vein of the Harry Potter and the Methods of Rationality discussion threads this is the place to discuss anything relating to Alicorn's Twilight fanfic Luminosity. The fanfic is also archived on Alicorn's own website.
Here is Part 1 of the discussion. Previous discussion is hidden so deeply within the first Methods of Rationality thread that it's difficult to find even if you already know it exists.
Similar to how Eliezer's fanfic popularizes material from his sequences Alicorn is using the insights from her Luminosity sequence.
The fic is really really good but there is a twist part way through that makes the fic even more worth reading than it already was, but that makes it hard to talk about because to even ask if someone is twist-aware with any specific hints is difficult. The twist is in the latter half of the story. If you are certainly not post-twist and want to save the surprise, then you should stop reading here and fall back to Part 1 discussion or to the fic itself.
If you think you're pretty sure you are post-twist and are safe to read the rest of this, try reading this rot13'ed hint and see if what you've read matches this high level description of the twist...
Rqjneq unf qvfpbirerq gur frperg gung Vfnoryyn jnf xrrcvat sebz uvz "sbe uvf bja tbbq" bhg bs srne bs Neb ernqvat Rqjneq'f zvaq. Va gur nsgrezngu, fbzrguvat unf punatrq nobhg gurve eryngvbafuvc gung znl unir pnhfrq lbh gb pel sbe n juvyr, naq juvpu znlor urycf gb rzbgvbanyyl qevir ubzr gur pbzovarq zrffntr bs YJ'f negvpyrf nobhg "fbzrguvat gb cebgrpg" naq "ernfba nf n zrzrgvp vzzhar qvfbeqre" naq gur jnl gurl pna fvzhygnarbhfyl nccyl gb crbcyr jub unir abguvat zber va gur jbeyq guna fbzr fvatyr crefba jub gurl ybir.
If the answer to the hint is obvious, then just to be sure that there is not a double illusion of transparency at work, here is the cutoff point spelled out explicitly:
Gur phgbss cbvag sbe cbfgvat urer vf gung lbh unir ernq hc gb puncgre svsgl svir (va gur snasvpgvba irefvba) be puncgre gjragl rvtug ba Nyvpbea'f jrofvgr jurer Rqjneq jnf cebonoyl vapvarengrq, Vfnoryyn fheivirf na nggrzcgrq vapvarengvba, naq fur unf gb ortha gb jbex bhg jung gb qb jvgu gur jerpxntr bs gur erfg bs ure "rgreany" yvsr.