I was wondering something. What happens if you start out as the only outbound back link on a PR 5 site, and then 20 other people put back links on that same page? Does it ruin your back link or make it less useful? If so, why would people bother sharing do-follow back link sites if it hurts their own back links?
Page rank is divided between the number of links on the page... i.e. a page with only 1 link passes 20x the PR as the same page with 20 links. People link to other sites for a number of reasons - link exchanges, paid for link, links to other sites they own, believe it's a worthy site, etc. A site that never links to any other site (PR Hoarder) could also trigger some filter because most sites have some natural outgoing links to improve visitor experience.
If the page that your link is on has any pagerank at all, then the more outgoing links on that page will just diminish the amount of pagerank that your page will receive. I wouldn't say that it makes your link less useful from an indexing standpoint, especially if the linking page is relevant to your content. They're not greedy? =)
Most of the times if you would look at the page that has outbound links only will always have NoFollow attribute (who are providing free links). The rest people will not give you out the links from the home page more than 100 links on a single page. Mainly they take care of home page. If the resources page has been created then its no problem for outbound links more than 100.
it will help you to gain the PR value of that page and similarly to the other sites too. thus there will be a PR leak distribution between all the links. Also, take note into the relevancy of the other links. if they are unrelated then value to your site link will also drop....
Usually because it's not THEIR site that is the PR5... they're just selling the information about WHERE to put the links to people who don't realize the usefulness will be "diluted"