There are some great hotels with the most awful reservations departments. The Thai Four Seasons properties come to mind...

So, if I can pay the lowest published room only rate, get all of the Preferred Partner benefits, and NOT have to interact with the in house reservations department, it's obviously much better to use an agent.

I only book directly if I am already negotiating with a property, or for simple stays at properties not affiliated with any consortia or preferred partner program (e.g. a Hyatt that is not Virtuoso or FHR). Otherwise, if paying published rates at high-end properties, one is usually leaving cash on the table by booking directly.