I though Google acquired Feedburner to expand their advertising empire but there’s another interesting thought:
Thousands of blogs are created everyday just to publish AdSense or run any Affiliate Programs. People also create Slogs to link back to good sites. Splogs gain high Page Rank rapidly as the sploggers put Feeds of good sites there and the power of content adds to the PR value.
One easy way to point out Splogs could be feed subscription count and, perhaps, that is the reason, Google has acquired FeedBurner. Link
BTW, Google already has the details of Google reader subscriber counts for every feed so that should also give them a fairly hint about splogs.