I think we're seeing more of one another's raw emotion because, as others have commented, it's easier to send an e-mail or post a blog entry than to write a letter. As a result, senders don't respect e-mail much as a form, nor do they respect its recipients. As you say, Tess, letter writing used to require people to ensure their arguments were logical and persuasive.
As a writer, I'm always mindful that--be it by letter, e-mail, blog, or speech--I'm communicating with people who deserve as much respect as I'd like to receive. That said, if I receive angry messages founded on faulty logic, I'm within my rights not to respond. Some angry senders don't care who/what's right; they only care that they get under your skin.