Of course any kind of encryption does make a dent in Dropbox's margins, since Dropbox's model is to dedupe data across all its customers but yet charging everyone as if the space used is strictly by their data alone. But the follow up question would be how much of personal (non-public and non-shared) data do people store vs. how much publicly available or shared data (not necessarily free) data they store in their Dropbox accounts for this to make enough of a dent.
