Why would anyone want a 'PR' button on their site? Do people really think PR actually means anything to the average visitor? Do they think PR really affects how that page ranks for a particular keyword phrase? It's pretty much just a measure of inbound links for the page but has very little, if any noticable, affect on how that page ranks for a particular keyword phrase. It's one of literally hundreds of factors that figure into the ranking algorithm.
That being said, I'm not sure how placing a PR button on your site would cause you to lose PR. Anyone who wants to know the PR of your page needs only to add the Google Toolbar to their browser.