To add some weight to the "PageRank leakage" theory, take a look at the following *images:
Image #1:
Image #2:
Image #3:
These images seem to lend credence to the "leakage" theory. If you link to a bunch of sites and don't get good reciprocal links in return, your PageRank decreases. On the other hand, if you link to sites with high PageRank and they reciprocate with links from high PageRank pages - your PageRank increases.
Again, I am still in the "guessing" mode, as many of us are (which keeps Google happy). So, I am definitely open to other opinions. That's why I posted the question. I don't want to just start deleting links to pages with low-quality reciprocal links if I don't have to. However, I don't want to give away all of my PageRank for nothing either - and that's exactly what I am doing based on the images above.
*Source of images:
http://www.iprcom.com/papers/pagerank/