Well, there are three methods in Opera (that I can think of at the moment) to access bookmarks - through the main menu - which takes a few clicks and maybe modifier keys, through a dedicated toolbar which takes up screen space (I sometimes work on my 1366x768 res laptop), and finally through the side panel which again will take an F4 (I don't usually keep it open) and a few click to get there.

Compared to a simple ctrl+T or mouse gesture plus a click, or the simpler ctrl+the speed dial number, it's too much of a hassle to use bookmarks for the most commonly accessed websites. In short, I'm kind of lazy.

In bookmarks I usually put sites I might want to come back later (but I rarely do), usually documentation or stuff like that.