The subject line is indeed the most important piece of information in an email message.
Therefore it is the first and only hint to what your message is about. While on the other hand a letter is where the body is in full view of who ever is reading. There are people who actually get hundreds of e-mail messages a day believe it or not. They can't possibly read them all ya know. So if your subject line does not seduce your readers. Than what is the point?