PR is important for a site in my view, it is not everything but it certainly is important.
If for example your personal blog is PR7 and you write an article based on certain keyphrases, you are most likely to be ranked very high for these terms.
One Man’s Blog - Specialization is for Insects. is a great example he posted this blog post: Buy Cheap Viagra And SEO - One Man’s Blog
and within days he ranked very high for buy cheap viagra and he kept the rankings for a long time. His website was in no way related to Viagra, but still the high PR of his blog makes it easy ot rank for whatever he wants.