Most OSes don't swap their disk cache, since they assume that reading a page from swap is no faster than reading it from the filesystem.

Correct. Moreover, disk caches in memory don't persist across reboots, which means the first access since OS startup is ALWAYS slow. You also can't return from an fsync() if writes have only been written to non-persistent cache. A persistent cache therefore can help even if you have oodles of RAM. Moreso if you don't.