A small (512 MB) swap partition gives you enough runway to warn on 25% use, alert on 50% use, and address some problems without the fun of abrupt shutdowns when allocations fail (or the OOM killer shows up). Monitoring for high swap I/O makes some sense, but 512 MB fills up fast, so chances are it'll fill up before anyone can respond to an alert in that case.

At least in my experience, it's pretty hard to actually gauge memory use, but swap use makes a reasonable gauge most of the time. There are certainly many use cases where the swap use ends up not being a useful gauge though.