Why would they implement a feature that's in every browser anyway? They should just design the site so that changing the font size through the browser works.
Buttons to change font sizes are anywhere in a browser's default chrome, therefore 99% of people don't know you can do it.
In general, I think it's dumb, but have been on the receiving end of "add font size buttons". No amount of "but you can just hit ctrl-+" does any good.
I believe his point was that a web design should be resilient to the fact that users can do this with their browsers. Among other things web specific I assume. That seemed pretty obvious to me anyways.
