Glenn Fleishman: After analyzing logs, I discovered that a small percentage of aggregation sites and aggregation servers were requesting as much as 20 to 30 percent of the bandwidth unnecessarily through aggressive downloads that didn't check the If-Modified-Since headers or other tools to prevent a retrieval of a page that hadn't changed.
Source: BoingBoing.
Randy: Glenn is finding out that well behaved aggregators mean that RSS scales.