sometimes the System scrollbar feel very wide and big if you have a slim container that have a scrollbar.
Or it can steal focus from the content it self, if your website is a darkly colored one and you have massive light grey scrollbar in middle of the site.
Sometimes the scrollbar is big and wide for a reason. I recently was asked to find out the registry settings on Windows CE to make all scrollbars forty pixels wide so they could be used effectively on a cheap resistive touchscreen.
Or it can steal focus from the content it self, if your website is a darkly colored one and you have massive light grey scrollbar in middle of the site.