Well dang. Now that I understand it I wish I would have done it sooner! Is it ever not cheaper? And it sound like it may be risky in a sense that you may not get your reservation?
Renting through a broker like David's helps remove a lot of the risk. And if you screw a broker like David's you can pretty much only do it once, they won't work with you again, and I wouldn't be surprised if they put the word out to other brokers.
As far as "is it never not cheaper?" I found, much like
@nickys mentioned earlier, that renting 1BR units doesn't save you as much, especially if Disney itself is offering deals, because the point values of 1BR units is often at or near double the point values of studios, but booking the room direct for Disney isn't double the cost. If you're sticking to studio villas, I think it's always a saving to rent points, except maybe when Disney is offering something like a free dining plan (that could also vary depending on how many people are in your party, clearly free dining for a party of 4 has more value than a party of 3 or 2 or 1).
Just an example for chucks and giggles - I picked a week in August at Kidani Village. Studio rooms are unavailable/booked so I couldn't compare those. WDW is offering a room-only discount which, with tax included, is about $608 a night for a 1BR savannah. Renting from David's that same week works out to about $553 a night. Saving $55 a night isn't nothing.
Same week in a 2BR savannah unit direct from WDW with their room only discount is $902 a night. Renting from David's is about $725. More substantial overall savings...asuming of course a 2BR is what you need.