Here's what I have taken from recent events. It may not be 100%.
Post-Panda, subdomains are being used by websites to host content that they don't necessarily vouch for. I take that to mean that if you want your root domain's authority to carry on, use a subdirectory. Authority will be extended to the subdirectory pages. If you want to absolve your domain of any infractions by the new content, place it in a subdomain. Authority will not be extended to the subdomain pages.
1. If you already have subdomains/directories and they are treated as separate sites: keep them no matter what. 2. If you don't have subdomains, don't try to make them, because it is not gonna work especially if you plan put there some kind of mashup of your content. Eventually it will lead to some reductions of traffic aka pandalization.