It's a completely valid argument, you've just misunderstood it. My argument isn't that there isn't enough screen space for more buttons, it's that if you put too many on there, especially buttons that most people will very seldom use, then the interface becomes cluttered and it becomes harder to easily hit the buttons you actually want to use. And as I said, for the vast majority of people that are sophisticated enough to use tabs, they'll almost all also be sophisticated enough to want to use a key command over an on screen button. So there are very few people who would actually want or use such a button.
I guess there's no huge reason to not make it as a customizable button. Perhaps they don't want to put in too many customizable button options because then people (especially newer safari users) will overload the toolbar with unnecessary buttons and actually make their safari experience a bad one. I think for the small number of people that would want such a button that they are well served by using the plug ins that were mentioned above (saft or whatever) that already can be used to add that function to safari.
