SA is easy to integrate in a mail server but not "easier" than adding one line to the server configuration. SA lets many spam go through, it's a pain in the neck to fine-tune it and when you have a lot of spam coming in it uses a lot of CPU for parsing. With blacklist servers spam is dropped at the beginning, there's no further parsing etc.

Basically I don't know why anyone nowadays would use spamassassin (I've used it in the past) when there's graylisting and blacklist servers that work wonderfully with low overhead.