Recently I came across and article by Eric Enge with Matt Cutt of Google and he points out that page rank play an important role in how often your site gets indexed by Google.
"… the number of pages that we crawl is roughly proportional to your PageRank".
Another way to think about it is that the low PageRank pages on your site are competing against a much larger pool of pages with the same or higher PageRank. There are a large number of pages on the web that have very little or close to zero PageRank. The pages that get linked to a lot tend to get discovered and crawled quite quickly. The lower PageRank pages are likely to be crawled not quite as often.
Also SEOMoz has a nice complementary article with graphics explaining in more detail what Matt is talking about.