Hi Lynton,
The idea is that people need to read the whole article before liking it or tweeting it, it doesn't make much sense to like an article if you have not read it yet, I don't think many people would do that unless they are trying to manipulate the number of likes or tweets.
The main reason for using an excerpt in a feed is because you want to make people come to your site to read the full article, so the excerpt is a tease to get them interested enough in the article to visit your site in order to read the rest of it.
Also feeds may be processesed by the consumer of the feed ie the feed reader, and they may remove some things like javascript or iframes so even when those things are included in the feed content your mileage may vary depending on where the feed is used.
Hope that helps,
Joe