I think some people just don't even know emails have a subject line. It also explains why people (having forgotten my email address and not saved it anywhere) will dig up an old email from me, reply to it with their new stuff, and end up sending me an email with the subject line "Re: Really bad news!" and then the body of the email is "Picnic tomorrow at 4pm!"
I can't stand that. If I do happen to forget someone's email address and have to dig up an old email, I'll delete the entire conversation and change the subject. I'm not continuing the thread of really bad news, I'm starting a new conversation.
Sometimes I have a hard time coming up with a subject line in my professional emails (new programs starting up, emailing clients, no set protocol for it), but I would expect that they'd read the subject line.
Honestly, I read ALL the subject lines in my email. That's how I prioritize what I read first. When I'm out for a week and come back to several hundred emails, I have to have some way of sorting through it all, and without the fairly standardized (within our office) method of subject lines, it would be nearly impossible.