There is a lot of webmaster interest these days in the impact schema markup has on content in search results.
Today's Webmaster Help video from Google addresses video markup. Matt Cutts takes on the following submitted question:
Rich snippets are automatically added to SERPs for video results from YouTube. Is it recommended to add schema video markup onsite in order to get your page w/embedded video to rank in SERPs in addition to the YouTube result or is this redundant?
Cutts says he checked with a webmaster trends analyst, and they said, "Yes, please get them to add the markup."
There you have it. The answers Cutts gives in these videos aren't always so straight forward, but this pretty much gives you a direct answer, and one which can no doubt be applied to other types of content beyond video. Use as much markup as you can, so Google (and other search engines) can understand your site better.