I've been wondering *something* similar.
If you have a user content area, say a forum added onto the website. Obviously many of the threads are not going to have external links pointing to them. Does that in itself hurt the website?
For example, I have a site that is 1,000 pages content created by me. Most of those have links to it. Then there are around 50,000 created by users with only maybe 1,000 of those having external links pointing to them.
Does that mean the user content area is hurting the entire website? If it is, that's ridiculous. As well, if it is, would the person be better off putting the user content area on a subdomain and not linking to it from the main website?