Google knows that removing the PR display would rocket through the roof the buying of PR. Perhaps that explains the three month update time. Compared to monthly display updates, and to removing it completely, the three month cycle probably leads to the least PR buying...
Care to share your logic on this? If I can no longer see a "PR8" on a particular widgets.com site, how does that motivate me more to buy a link from them?
I'd argue that instituting a delay of several months will probably generate the most link sales demand, since the thinking becomes "I'd better buy that link TODAY, to increase my own PR by Christmas".
I'd agree they'll always have internal PR in their algorithms, but I can't see how having any display of that PR to SEOs helps them in the long run.