Googler, Susan Moskwa explained why in the thread:
We've been telling people for a long time that they shouldn't focus on PageRank so much; many site owners seem to think it's the most important metric for them to track, which is simply not true. We removed it because we felt it was silly to tell people not to think about it, but then to show them the data, implying that they should look at it. :-)
Interesting... So you tell people not to focus on it and you find it silly that you show it in Webmaster Tools, but you still show it in Google's Toolbar? I mean, how many people have the Google Toolbar installed compared to those who use Google Webmaster Tools? I assume a fraction of those use Google Webmaster Tools.
Back in 2007, Google wanted feedback on removing PageRank from the Toolbar. I felt it was a good idea but the idea died out. Google cannot remove PageRank from the Toolbar, it is too much of their branding. No matter how much Matt Cutts and the Google search quality and webmaster trends team want it removed, I cannot see Google's executives allowing it.
Removing it from Webmaster Tools does make a statement to webmasters, but as long as they keep it in the Google Toolbar, it makes a stronger statement to searchers.
Here is how the PR stats looked like in Webmaster Tools:
Shouldn't Google drop it from both places, the Toolbar and Webmaster Tools? Having it public anywhere, defeats the purpose in my opinion.
Forum discussion at Google Webmaster Help.