Personally, the ability to fold the notebook over and easily tear sheets out even if the pages aren't perforated outweighs any inconvenience. Lefties complain about how the binding gets in the way and hurts their hand, and even righties aren't immune to this problem if, like me, you always write on both sides of the page and wind up with a right-sided binding when you flip the notebook. Some people love spiral bindings and others hate them. You can't get much more versatile or cost effective than the traditional spiral notebook.
When it comes to office supplies - as in, supplies that actually get used in the office - the traits I value most are (1) versatility and (2) cost effectiveness. Welcome to Part III of my ongoing series, 'Everyday Writers: The Best Paper for Everyday Writing,' in which I've been taking an extended look at the different types of paper I use in my daily life, with a focus on work.