Some people think that outbound links "leak" PR away from your page; other people think that they don't. The truth is somewhere in the middle.
Let's say that you have a site with two pages, index.html and products.html. Each of these pages links to each other.
With no other links, these pages share all of their Page Rank with each other.
If you put a link to www.example.com on your index.html, it will not "leak" Page Rank from index.html -- that is true.
But, it will halve the amount of Page Rank that index.html sends to products.html.
As a result, products.html will have less Page Rank. As a result, products.html will have less Page Rank to give back to Index.html. That will lead to index.html having less Page Rank as a direct result of the outbound link to www.example.com. Both of your pages will lose a small amount of Page Rank.
Interestingly, products.html will lose more than index.html. Can you figure out why that is the case? I've been recommending PageRank Explained with Bright Colours for several years as a good explanation for how the Page Rank algorithm really works.