How important are outbound links?

[BloodNinja3]

I recently started a directory on a domain i wasnt using.
When people add their site they are asked to link back to me but its not required. I want to keep my directory for the people who use it, i don't plan on charging for adding a site and i dont want to force people to link back to me if they dont want to.
So what im wondering is will the amount of outbound links do me favours or would it really be worth insisting on a backlink?
Comppared to backlinks, how effective are outbound links to the site that hosts them?

Also, i wrote a mod that displpays the last referrer in the footer of everypage.
Im hoping that if people have their link displayed in the footer of every page they might be more inclined to link back.