Always show a vertical scrollbar in Firefox, Chrome, Safari and Opera

I don't think I would force this on a published page either, since it's basically a personal preference.

But it certainly could be done by one user for his/herself. Safari and OmniWeb let you set default stylesheets, and Firefox will recognize one too if you create a [profile]/chrome/userContent.css file.

What makes it annoying?

(I'm asking out of genuine curiousity, not because I use it.)

It's a necessity if some of your site's pages aren't very tall and you use a centered layout. Pages shifting back and forth horizontally when navigating between short and long pages is annoyingly jarring.