Page rank sculpting is trying to change how the page rank flow within your site using various method like adding nofollow attribute etc. To understand how Page rank sculpting works first you need to understand how page rank works. This post by Matt Cutts who is the head of Google's web spam team has a good explanation about page rank and page rank sculpting.After going through the blog post what I felt was that the nofollow attribute is only useful for disclosing paid links. In the comments section Matt even encourages removing nofollow from pages like privacy policy and terms of service. Below quote from the blog post explains how page rank used to work and how it works now after the changes made by Google.

So what happens when you have a page with “ten PageRank points” and ten outgoing links, and five of those links are nofollowed? Let’s leave aside the decay factor to focus on the core part of the question. Originally, the five links without nofollow would have flowed two points of PageRank each (in essence, the nofollowed links didn’t count toward the denominator when dividing PageRank by the outdegree of the page). More than a year ago, Google changed how the PageRank flows so that the five links without nofollow would flow one point of PageRank each.

As you can see this is going to be a big problem for bloggers , especially for blogs that gets lots of comments. Even though you add nofollow to the links they are still leaking page rank. A possible solution is to have a third party comment system like DISQUS , Intense Debate which are based on iframe or java script. In one of my previous posts I blogged about the benefits of using DISQUS. Andy beard gives a much deeper look into DISQUS comment system and why a comment system like DISQUS is a good solution to the page rank leakage.