I did a survey last September -
http://carpejugular.com/rss-feeds/ - and from around 40 sponsors, only 8 provided feeds which validate fully: and that is before you even get into the question of which you might actually
want to use.
Quite a few will display without breaking your pages even though they do not validate, but if you are producing feeds, why take the chance for the sake of saving a few minutes work and getting the codes right?
Commonly too feeds are structured in a way which makes it difficult, even impossible for sites displaying them to format them to fit their designs. Some impose their own formatting onto host sites.
Between the poor code, dismal - in most cases - text and often third-rate content, RSS is something very few sponsors seem to take seriously. That's short-sighted if you consider how long it took adult to catch onto blogging and how successful it has proven in mainstream.