Before writing the body, it's a summary, so you should have an idea what you're going to say before you start typing. Think of it as a requirements document for the rest of the email.
But what if the the idea changes, as it so often does? I can't tell you how many times i did exactly this, and then had to change it later. You may call me fickle, though i think it has to do with "getting into it" and allowing things to change.
After the intent to write, and the writing itself, a summary would be accurate. As the ideas are already worked through, i think it a much better time to write a summary.
A good subject line also gives the reader a reason to read it.
The subject line gives a good reason to read it, but so does the sender. Indeed, i bet i read more email based on sender than subject line. Further, i find the subject line is much better to review when looking at old email, not new email.
That is, when i receive an email, i will most likely read it if it is from family or friends, regardless of subject line. I'm reading it--please excuse my disagreement with you--simply because they sent it. However, when i need to find an old email from said persons, who sent it is simply not enough (any more), a summary is, however. And not an intent of what they wanted to write, rather, what they actually did write.
If, though, the sender is unknown, the subject line is an indicator as to my want to read it. Perhaps then, the intent to write is better than the summary, as the function is now one of advertisement instead of summary for later review. Indeed, as these emails are not from family, friends, or soon to be friends, i am unlikely to be looking back to even require a summary.
I want to thank you for your message, it is well thought out. I happen to disagree, but i enjoyed reading and responding to it.