Pack weight is markedly reduced. Weigh a guidebook some time. Pages can be disposed of as they are used. It may be nice to have that guidebook on your shelf for reference and memories when the travel is done, but the lighter your pack, the fonder those memories will be. A mailing scheme is easier, cheaper, and more reliable.
Letters make it to places better than tamper-tempting parcels.
It would be nice if you could legally photocopy pages from guides that you find in the library. Often, libraries have lots of guides for an area, each with different advantages. Photocopying the sections you want could be cheaper than purchasing the guides themselves, but it is illegal. As a writer, I appreciate the spirit of this particular law. There's a lot more to the issue of guidebooks than many travelers realize.