In a lot of cases you only need to type the first letter then hit enter. And when you open a new tab the address bar is selected by default, so instead of moving your mouse to the top of the screen and finding the MacRumors bookmark, you could simply hit 'm' for MacRumors and hit enter. Another reason for not using them is that they take up space on the browser bar, meaning less space for your webpages.
and if it's in the bookmark bar in safari i press command + 3 and i'm off to macrumors ...so it's pretty even in terms of button presses / not needing to reach for the mouse
personally i use both