There are situations where acquiring a high PageRank link (PR7 and higher) can be a risk or be detrimental to the site that acquires the link. This post covers some of the potential pitfalls. The primary danger in the scenario where one might score a PR8 link (for instance) is the risk that such a link is published by the PR8 site, subsequently indexed by Google or Bing, and then for any number of reasons removed or no longer linked to by the PR8 site in question.
What happens in this case is that a strong vote for the site that was linked to is withdrawn in essence, as Google sees all links as topical votes for the topic of the site receiving the vote. If there are no other PR8 links pointing to such a site, it equates to the most authoritative link in the linked site’s link profile being withdrawn or it’s most influential vote of topical confidence being retracted. A PR8 link value (extremely conservative value) is that of about two dozen PR6 links, if the page factors such as number of outbound links, remain constant.
This means the removal of a PR8 link would equate to the removal of 25 PR6 links in one fell swoop. Unless the site losing the PR8 link has numerous PR6 links or links of that caliber, then this could prove disastrous.
If one acquires such links through a link network where links are published without first requiring publisher approval, the scenario described here could and does happen. Any automatic link publishing platform is inherently risky due to the lack of control on the part of the link buyer (renter) and the link seller. A link buyers link could be included in the company of any number of more-dubious links, such as those pointing to gambling, adult, replica watches, work from home or get-rich-quick schemes (just to name a few). In addition, links may even be placed on such sites. So, if the link buyer prefers not to be in such company or included on pages with undesirable content then such networks are to be avoided.
These are only a couple of problematic scenarios. I will cover more later.