On what basis do you make this assumption? The toolbar is not natively supported in most browsers and is in fact primarily available via third party toolbars that just happen to be SEO related.
When toolbar PR was introduced
The keyword in your statement is "when." Living in the present, all can freely see how pagerank is heavily relied upon to barter links. Pagerank values should be eliminated from the public not just because of its use in buying/selling links, but also because the data is old.
A lot more goes into page trust than an arbitrary number Google has assigned it months ago. For example, dropped domains purchased from auction can retain pagerank values. If "ordinary" users were somehow able to see the toolbar pagerank, they may actually be placing trust in a domain that no longer belongs to its original owner.
As I originally noted, Google's hypocritical stand when it comes to buying links is quite apparent. In this case, Google benefits from each toolbar export from the buzz (free marketing) it creates in the webmaster community.