Because it allows so much flexibility, it's difficult or impossible to standardize the width of the Sidebar. Way back in OS9, things were more rigid and did not change when you reopened a folder. With OSX allowing multiple Views and settings, Finder no longer saved the View for each folder, and modifications made to one window showed up when opening another (not settings, but things like window size).
See post #4. In those days, there wasn't a sidebar. I can't speak to OSX prior to Leopard, but my hunch is that the flexibility of window views was present all the way back to 10.1, along with the sidebar. OS9 would also remember each window's position on the screen. I had elaborate cascading folders, etc. All that went away when I upgraded to Leopard.