[joshzayin]

Personally, I have a password that my password manager generated that I use for it. I had it written down in my wallet for a while, but after typing it multiple times a day for a while I memorized it and since destroyed the paper. It's a shorter password than what I use for my stored passwords, but I think it strikes a good balance. (And it's not a GUID, but if you think you could memorize that then it probably couldn't hurt. That's risky, though -- if you forget, there go all of your passwords for everything!)

I don't know of any off the top of my head, but there was that time a few years ago when Dropbox accidentally let anyone in without a password. This isn't to pick on Dropbox, but security lapses happen and it's wise to have multiple layers of strong defense to reduce your risk. (Also, if someone compromises the email associated with your CloudDrive, they can use that to get your CloudDrive by invoking a password reset.)

EDIT: Wolfram|Alpha estimates the entropy of a password generated using the constraints I used for mine as roughly 85 bits (the relevant space would take 14 trillion years to enumerate). It actually has a pretty information-heavy password strength estimator (though I can't attest to its reliability as I'm not familiar with the internals).