Well they can be intercepted during transit, but that really is trying to shoot a moving target. They would also be either trying random IP addresses to try to monitor/intercept http/smtp traffic or targeting a specific IP address.#
All of this makes the actual chances of your email traffic http or smtp/pop3 a bit of a low risk.
I don't use webmail, generally only through an email client (currently thunderbird). I haven't specifically used SSL email in all the time I have been using the Internet (quite some time) and I haven't had an email account compromised (e.g. user name and password harvested).
Only recently since getting broadband one of my email accounts btinternet, which uses Yahoo to handle mail does it get sent using SSL. This email account however, I very rarely use as I too have my own domain name and email hosting for that and it isn't SSL either. I haven't considered upgrading that email hosting service to SSL as I personally don't think the risk great enough for the content of my emails.