P(your data compromised) = 1 - P(storage provider never hacked)^(number of storage providers)

All else equal, using more storage providers increases your risk of exposure, not decreases it. And for most classes of sensitive data, the loss of any of that data can still cause problems, e.g. imagine health records... if only 1/5th of your patient data was hacked, that would still be a very bad situation (and before anyone asks, yes people use Dropbox for this[1]).

[1] https://www.dropbox.com/static/business/resources/getting_st...