My main reason for not carrying them around is that I have several reading devices (such as an iPad and an iPhone), and I would find it super-incovenient having to separately load each and every book onto every device I own, having to worry about keeping the various devices in sync manually.
With cloud software like Dropbox, SugarSync, Box.net, Copy.com or Google Drive, you load every book only
once, and it's up to the cloud software to keep every book in sync on every device and computer. If I can let a machine perform something I would otherwise have to perform manually, I'll let the machine do it 10 times out of 10.
