I think we should not forget about this slide: http://www.extremetech.com/wp-content/uploads/2013/06/PRISM-...
so basically this move, in my opinion, just means that dropbox has been added to that very list and there is no point in using other providers shown as an alternative (e.g. google) or even just hope that dropbox will care more about privacy after she steps down eventually.
It's what the move symbolizes. If there is someone on board who pushed for warrantless searches/taps/etc and someone who probably would be in favor of PRISM and was in favor of past other programs, then I don't want to be affiliated with that company. And Rice was a public figure who spoke about the Patriot Act, searches, doing things in the name of "national security", wars, blacklisting etc.
That makes me question the person's morals, and shakes my belief in how the company will be run if someone on the board has these kinds of morals and will be making decisions that may affect my data and my person.