I knew this, but I’m posting it for posterity. Not just because Google said so, but because it makes perfect sense.
From Google’s tips on using AdSense in feeds:
Include the full content of your feed in your posts. Including more content increases subscribers, which in turn increases the number of impressions you serve into your feed. Though you might think this will reduce clicks to your site, our experience has shown this isn’t always the case. Often, the audience reading your feed is a separate audience from those who visit your site, so we encourage you to monetize your audience of feed readers separately.
So there. The monetary aspect of it is one thing, but the separation of your audience is the bigger issue, IMO. There are other things you might need to worry about besides ad placement to serve those separate audiences.
Of course Steve Pavlina, who just dropped his six figures worth of AdSense advertising all together, would say differently. If you’re making money, it ought to be because of the value you provide, not just the traffic you generate. Those are related, but not the same.