And, to some extent, that has proved to be true. Viruses have gotten trickier and spammers have gotten nastier. I think one of the biggest advantages of a system of this nature is that it would save a lot of system overhead. Though we've obviously seen a considerably advance in processor power, I still shudder at the thought of unzipping, untarring, ungzipping, un-Stuffing, uudecoding, etc., attachments to all incoming mail to run a full virus scan on every one. Some of my mail servers, I imagine, would gurgle, choke, and then die a horrible death at the hands of such abuse.
Free beer is good.
Given Symantec's patent on virus updates, I'm a little skiddish about the whole anti-virus industry. A community effort is, to me, quite preferable. While this capability now exists, many mail systems are still on 8.9 and earlier and will not be able to take advantage of sendmail filtering. Also, this is simply an API, and so relies on other offerings like vbsfilter to actually perform the filtering. The alternative solution is to set up procmail as the local mailer, and do this filtering via procmail rules.