How about something like this for the grandparent's case:
"In some countries people can be persecuted for their religious beliefs, politics or sexuality. Their text messages could potentially reveal all of the above to the government and/or someone listening in on GSM traffic [1]. Those people could use an SMS encryption app to provide them with plausible deniability or, should their local law doesn't have that notion, at least help them avoid automatic keyword detection."
The problem seems to be, to me it's so obvious that it matters even if you have "nothing to hide", that whenever someone confronts me with that argument I'm a bit dumbfounded :)
"In some countries people can be persecuted for their religious beliefs, politics or sexuality. Their text messages could potentially reveal all of the above to the government and/or someone listening in on GSM traffic [1]. Those people could use an SMS encryption app to provide them with plausible deniability or, should their local law doesn't have that notion, at least help them avoid automatic keyword detection."
[1] See, e.g., http://www.ti.bfh.ch/uploads/media/19_Vadym_Uvin.pdf.