I think the centralized swap lets people get exactly (and only) those seeds they really want.
But it takes some paperwork and an angel to volunteer to be the central person.
The only Round Robin I was in had TONS of seeds when it reached me - but mostly no-name and/or very cross-pollinated with unspecified varieties. I think it was the "LARGE" flat rate USPS box. POUNDS of harvested seeds! The recipients had to scoop out individual servings and write out a label for their little packet.
Then someone got greedy, and people after her got very little variety and much less quantity.
Banning her after the fact didn't help the rest of the people in that RR.
It wasn't THIS website!