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Once someone has access to your e-mail, they can reset your password and log into anything that you used that e-mail to sign up for. With Dropbox, all they'd have access to is whatever files you happen to have in your Dropbox at the time.

1 reason would be that you can reset your dropbox password with your email, but you can't get your email password from dropbox (unless you store something silly like passwords.txt in dropbox)

Email is basically master key for all accounts.