I have written one (or two) in my blog. I now feel very ashamed about it. Every open letter I read makes me cringe. Open Letters are usually defined as letters read by everyone else except the intended recipient. Mainly because the writer does not have the 'vakku'/ is not important enough to attract the attention of the intended recipient.
I think Rediff started it and Internet based writers use it a lot. The main reason people write it is because they want to say something to someone important. Since the important person is never going to read it, the writer wants at least someone/anyone to read it.
Futility. Desperation. Sigh.