I don’t know what the argument is against showing favicons in Safari’s tabs, but I can only presume that it’s because some contingent within Apple thinks it would spoil the monochromatic aesthetic of Safari’s toolbar area. I really can’t imagine what else it could be. I’m personally sympathetic to placing a high value on aesthetics even when it might come at a small cost to usability. But in this case, I think Safari’s tab design — even if you do think it’s aesthetically more appealing — comes at a large cost in usability and clarity. The balance between what looks best and what works best is way out of whack with Safari’s tabs.
As I tweeted, John's plea is a prime example of "accessibility for everyone." The idea is the colorized glyphs along with the text make switching tabs easier because the icon is an additional visual cue that helps processing.