I loved the library at first, but it became clear early on that the library was not a securable location.
It also occurs to me that those hard library chairs must have been hard to sit on hour after hour, take after take... Didn't they use the Torrance High school's actual library for the first season? I could see that being a nightmare. A lot of the first season sets I think were in actual spaces, and moveable sets only got built later on once the show was not going to be canceled.
I loved Giles's flat because it was cosy and felt home-y. I enjoyed Revello Drive but I never saw it as scooby central, even though it did become that in the final year, I think Buffy had enough. I think it was too much. Earlier, I think the fact that Buffy's mum didn't know about the slaying for the first couple of years made it easy for them to avoid it as a place to center research and Giles's place was a good fit because like Buffy's house it required permission and probably could be magically protected. Also, the fact that his library of books was at his house was a very good thing. I think there's convenience involved with using the magic box, but security isn't there. Would Willow have had as easy a time breaking into Giles's house if the books had been there?
Anyone could have accessed those dangerous books, kept right there in the magic box. It's madness.
I would not have minded the flat being scooby central later on, even after the magic box was used for training. Once Giles left, there's a need for the central location to be elsewhere but before that, I'd have strongly preferred the privacy and securability of Giles's home...
From his perspective, however, I could see him NOT wanting to have people tramping through his house at all hours and interrupting any chance he had for a private life. I do think that all those dangerous books in a place anyone has access to doesn't seem smart.
Even his flat wasn't as secure as it could have been, but at least, it wasn't a public place.