I've used snail mail quite a bit. One or two love letters. And using Freedom of Information to expose my local council as lazy bureaucrats. But I once heard someone say, you should never assume someone else is going to congratulate someone on a good job, or console them if they're down - it's the same principle as calling an ambulance; do it, even if you're sure someone else has done it beforehand, there's a chance they might not have. To that end, I once wrote a hard copy letter to George Galloway (the disgraced MP / freedom fighter).
But someone in chokey? I'm not so interested in the average prisoner, but I think it would be fascinating to write to a terrorist (assuming the prison-penpal system redirected your mail so that he or she never learned your home address). Let's be honest, we've all got our doubts about democracy, and the West. I've got zero justification for how anyone in our society gets away with what they get away with, only a certainty that the violence is wrong (but again, for reasons I can only get to the bottom of while thrashing it out with a conversation partner) -- therefore I think it would be therapeutic for both me and the crim.