First the misconception is Google PR updates are very frequent but the toolbar PR is not very frequent and the main reason is There are thousands of pages added every day on internet and new sites pages links to many pages and so it would not be fair to update the PR less frequently. Now if Toolbar PR is updated very frequently it would lead to more people tend to assuming lots of things in the Google SERP and it can be manipulated to a large extent.
Exactly. Frequent PR update in toolbar makes people aspire after PR value blindly and ignore the contents of web pages.