You paid for it when you signed up, so you are entitled to use it. Modern mail programs already allow you to set up multiple profiles for roaming, which include matching mail servers with dial-up connections. Yes, spammers will begin using their ISPs' mail servers again. However, as you pointed out, the ISP now has solid evidence to ban that spammer and even put a restraining order on them if necessary. E-mail is hardly private.
Mail servers transmit e-mail as plain text and store it as plain text.
If you are concerned about your mail flowing through your ISP's mail server and being opened by its administrators, you should use PGP . I don't believe anything should stop a user from using an alternate reply-to address when sending mail. Even so, we can stop unauthorized relay attempts through our server and stop trespass spam from our dial-ups. We allow mail relay either freely from within our own dial-up networks regardless of who it is, or from outside of our network if they first check mail with POP3 protocol. Nothing about valid return addresses, non-local return addresses, or any of that, as we could care less.