From Caleb Crain at The New Yorker (http://www.newyorker.com/online/blogs/books/2014/05/the-new-york-public-library-comes-around.html):
Last week, the Times reported that the New York Public Library, in a surprising about-face, has given up on its plan to tear seven stories of bookshelves out of its white-marble flagship building, on Forty-second Street and Fifth Avenue. The bookshelves, usually referred to as “the stacks,” literally hold up the palatial reading room on the library’s third floor, and, in 2008, when the plan to remove them was first announced, they contained the heart of the library’s research collection. . . .
Almost every major research library has to store part of its collection remotely, but in this, as in all things, there must be proportion. It takes time to get a book from New Jersey into Manhattan, and if a researcher has to wait a day or two to see a new text some of the serendipity goes out of research. If a researcher’s deadline is tomorrow, a book that he can’t see until the day after is of no use to him.
I would add that, for out-of-town researchers planning a trip to an archive, this access is critical as well. The best-laid plans and online searches of collections can’t prepare you for something that you discover on site, and, as Crain says, some of the serendipity goes out of research if you can’t access the texts before a 24- or 48-hour (or longer) wait.