SpamAssassin is the Swiss-army knife of spam filtering. By encompassing other technologies such as DCC and RBLs, coupled with highly configurable rules-based scoring, it holds an edge over most alternative spam filters.
However, there may be benefits to using more than one spam-filtering tool:
If system load on an email server is high, a separate machine hosting a simple filter can filter out obvious spam, allowing further filtering to be carried out using SpamAssassin. ISPs often take this approach, filtering some spam, notably pornographic spam and verbatim copies of known spam, but avoiding statistical analysis, which is computationally intensive.
A spam filter that uses a different statistical analysis method than SpamAssassin will be triggered by different spam. This should produce a higher combined filtering rate.
By using two filters, spam detection policies become more flexible. At the simplest level, emails can be considered spam if and only if...