Scoble explains why you shouldn't use partial text feeds:

Out of, say, 1,000 people who are on the Internet, only a small percentage read a lot of feeds. Let’s say it’s 10%. That means only 100 out of any 1,000 people will read feeds and of those 100 people only a small fraction will bother with ZDNet’s feeds.

The thing that partial texters are forgetting is that the other 900 people will find out about you from an influencer. Someone who will tell them. So, your traffic growth will be far slower if you only offer partial text feeds. Many of my friends who are journalists or bloggers just won’t deal with partial text feeds anymore.

Here's how people who want to pitch full feeds should go about it - describe them as a loss leader. Sure, the small percentage of people who find you via RSS/Atom won't see your ads. However, the people who follow their links will. It's no different than offering a sale on some small item in order to suck people into your store - you'll make up the loss on all the other sales. With syndication, it's even better, because - as Scoble points out - there aren't that many people (relative to the whole audience) reading you that way. They happen to disproportionately be influencers though.