Don't take pagerank at face value in your SEO
If you are doing SEO for your own or someone else's website, you know what pagerank is. You have heard people claim that they use pagerank as the primary or even the only basis for their ranking. They focus on building links from high page rank sites and use pagerank as one of the only aspects of their keyword research strategy.
There are a number of reasons why pagerank is not an effective way of measuring the competitiveness of a webpage. You can read more about it in Why Toolbar Pagerank is Worthless.
Pagerank varies significantly on the front page
If pagerank was the most important part of SEO, you would expect that all of the results on the front page would have about the same pagerank. As you would move from to the second and third pages, pagerank would decline and still be relatively consistent. A couple of experiments I ran showed different data. I used the Google Bulk Pagerank checker on the keyword "technology startup" and found that the pagerank values of the pages varied between 2 and 6.
I also conducted another pagerank test after using the keyword "social bookmarking." A couple of the pages had no pagerank at all, while a PR 10 page (the page to the Add This! widget) wasn't even on the first page.
The pagerank that is given to us is an illusion
Google updates its pagerank toolbar infrequently. The last major pagerank update to the toolbar was over eight months ago. Although Google still updates pagerank for its algorithm, it doesn't share this information with us right away. If you are trying to build links on blogs that are listed as having a pagerank of 4, then you may actually be putting links up on a page that has a pagerank of 1 or 2.
Obviously, this can result in a lot of wasted time. It may make more sense to identify what pages are likely to be popular in the future and build links there. Being the first to put a link on a high page rank page has the added benefit that it will be the first to be seen, which can also generate a higher level of click through traffic.
Pagerank causes site owners to be afraid to use external links on their page
Readers come to your site for quality information. Limiting the number of quality links because you are afraid to lose your pagerank is a great way to lose traffic and decrease sales. You are building a website to make money and get visitors, not to build pagerank.
Don't forget about other ranking factors
Google has just made some changes to its ranking algorithm. Matt Cutts says it is now based more on trust than pagerank. Pagerank may soon be an obsolete metric for SEO. Don't obsess over something that is unlikely to give as much benefit to your site as factors such as domain trust, relevancy and anchored back-links.