This is a bit of a sensitive issue with a lot of people. The majority of inexperienced users would probably welcome spam and/or virus filtering on their email, but almost all advanced users are strongly opposed to any kind of mail filtering done by the ISP. The job of the ISP's mail servers is to accept the customers' mail and make it available to them, without unnecessarily tampering with it. Some users may *want* to receive spam emails or viruses, for whatever reason, and it is not the place of the ISP to say that they cannot. Also, no mail filtering software is 100% accurate, and you can guarantee that some legitimate email would be filtered out by the system.
Installing spam and virus protection is the responsibility of the user, and the most the ISP can do is educate the user about the need to do so. I run my own mail server, so I don't rely on ntl's mail servers for my main email, but if I did I would have to seriously consider switching ISP if they started filtering email.
That's the way I see it, anyway.