I'd rather get 1000 junk messages than lose one valid one
I have perfectly functional spam filters that take care of all the junk that comes my way, but there is no way for me to recover mail that does not arrive.
Thus, I was aghast to find that my ISP clandestinely censors my mail according to a blacklist of IP addresses maintained by a third party. What is more, even when I informed them that mail was not arriving they refused to un-block my associate's address, insisting instead that I/he would have to deal with the third party to have the address scrubbed.
I'm paying my ISP to accept mail on my behalf - not to censor it based on origin. For them to discard something - anything - without my consent and based on a third party IP address list - is like a post office throwing away all letters that come from Nigeria because they've been told by someone there a lot of 419ers live there.
As for true spam filters... certainly delete anything that contains V!4GR4...