Ironically, for me, paying for Dropbox is a really bad idea. One of my main uses for Dropbox is sharing files - copyrighted files, usually, especially with the Research page's various requests that I have fulfilled. Knocking out one Dropbox account knocks out all its files, so I want to spread files over as many Dropbox accounts as possible. Paying for extra space just increases the temptation to put a great many eggs in the one basket.

(Of course, no actual problems have popped up over the past years I've shared files on Dropbox, so there's no point in having too many accounts; right now, I just shift Dropbox accounts every year or two. It is a real problem, though. My favorite Vocaloid music site, mikudb.com, recently saw its main uploaders' account on MediaFire disabled, which broke the availability of ~1300 albums.)

EDIT: these days I don't use Dropbox as heavily as I used to, as I am more comfortable with hosting files on my own website: the bandwidth bills are not as bad as I feared, and experience has shown thus far that I don't need to worry about legal reprisals as long as I'm not dumb about it. I still split my uploads over 10 accounts, though, and rotate.