[angch]

Like a highway brake failure ramp, you have room for handling failures gentler. So services don't just get outright killed. If you monitor your swap usage, any usage of swap gives you early warning that your services require more memory already.

Gives you some time to upgrade, or tune services before it goes ka-boom.

[akerl_]

If your memory usage is creeping up, the way you'll find out that you need more memory is by monitoring memory usage via the same mechanisms you'd hypothetically use to monitor your swap usage.

If your memory usage spikes suddenly, a nominal amount of swap isn't stopping anything from getting killed; you're at best buying yourself a few seconds, so unless you spend your time just staring at the server, it'll be dead anyways.