I don't really think the We are going to be a very friendly library staff that serve as the basis for a long term institution. They can move books around, sure, but will they be able to properly assist users of the library? Will they go out of their way to help people hunt down knowledge? Will they be loyal to the idea of perserving knowledge, not for themselves but for future generations of species they have nothing to do with?
These are very alien minds and I don't think they'll produce a library that is good and useful for humans, dwarves, and halflings. It seems like a "cool spiders" choice rather than that they will make good librarians in what is ultimately at least half a service position.
I think we should keep in mind he kinds of priceless treasures we are going to be putting in this library. There will be books in there on necromancy and vampires, magic books that could be misused, dwarf books that reveal parts of their society they do not want to share with most people. All in all I would rather those books be safe than having the maximum smiles coefficient. I mean just the magic books alone, who would you rather have guard those, random humans who might be tempted to do something silly or the spiders with no capacity for or interest in magic?
There are solid practical reasons for the spiders, also they are cool