Publishers who depend primarily on advertising to make a living should offer two feeds: a summary-only feed without ads and a full-content feed with ads. That leaves the choice up to the user.
I think it's possible to enable this with only one feed. Summary on RSS description element, full-content with ads in content:encoded. Bloglines gives users an option for them to choose between "display entire posting" or "display summaries (if available)". All RSS aggregators should provide this kinda option like the way Bloglines do.