The more that you bind the faster your reaction times will be. Bar none. If you need a Frost Nova or an Iceblock NOW you need it NOW not once you've scrolled your mouse over to the icon and clicked the correct one.
Clicking will also have your eye tracking the mouse cursor and you will be less attentive to changes on the battlefield. If you are trying to find the right spell amid a sea of icons you'll miss the errant mob that somehow got loose and is about to turn your face into dinner.
That being said, I don't have all of my spells on my action bars. Some things don't need to be accessed in the heat of the moment (like professions or Portal spells) and thus can be left in the spell book.
The rogue forum has generated
this fantastic post about key bindings which is a huge help in understanding why key binding is superior and how to go about setting it up. Personally, I use the addon Bongos to easily bind all of my keys. That can be found [link=www.wowinterface.com/downloads/fileinfo.php?id=4744]here.[/link]