@splashingpixels.com – Yes, that’s true, but as Tevya points out it’s sometimes very desirable to be able to globally disable comments rather than having to set it granularly, especially if a number of posts, pages or products have been entered with comments/reviews enabled.
The other issue is that some themes will display a comments or reviews tab regardless of whether you have comments/reviews enabled on a per product basis, and globally disabling comments/reviews will make this tab disappear (on well designed themes). I personally dislike having a content tab that will never have any content in it, and disabling via CSS or in the template.php file is much less desirable than just turning off the feature globally.