The NSA is only one fear - there are other actors you'd expect to be doing similar things. e.g. Chinese, mafia.

Using things like Blackphone can potentially increase the cost of anyone doing this kind of spying, to vastly reduce who will do it for what reasons.

This talk by Dymaxion is good on economics and usability of this stuff: http://dymaxion.org/talks/EaPitLW.html