In most non-casual games it's all about maximizing how many commands you can issue. WoW (and SC2) have interfaces for which a large part of the interaction is o

In most non-casual games it's all about maximizing how many commands you can issue. WoW (and SC2) have interfaces for which a large part of the interaction is only efficient at all with the mouse. If you then depend on the mouse for other things, you've created a bottleneck. All good WoW players bind all their skills to keys rather than clicking them because if they didn't do that they couldn't use that skill while running, or while selecting a new target. It's not about the cost (or lack thereof) of mode-switching between keyboard/mouse, or the cost (or lack thereof) of consciously manipulating the UI -- it's purely about being able to walk and chew gum at the same time.