Drag and Drop takes the place of "save to disk" or "send to device" It does not physically remove the book from your Calibre library. It's just a quicker method of getting books out. Using the dropbox as a vehicle to share books, pictures, and other large files with friends is great since you can assign access to different people on a folder by folder basis. And with the Kindle, you can quickly download the book onto your Kindle directly from the dropbox, using the Kindle wifi browser.
The other discussion has been about "moving" a Calibre library from your hard drive to the dropbox so that you can access it from multiple computers. The plus is easy access from more than one computer, the con is that you need internet access to get to the dropbox and your Calibre library. I personally came to the decision to use the dropbox as "cloud" backup for my Calibre library and to leave the working library on my hard drive.
Hope this helps explain. Yell if you have further questions.
cheers
my dropbox referral link:
http://www.dropbox.com/referrals/NTMzNzQ2MjE5