They must understand that user can (and should be able to) change (just the) font size, if not their designs are often useless.
How many sites can you really change the font size? Unless of course you mean through the browser.
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.
What's the percentage of users who manually change their font size? Is it the majority? Because if it's not, then I'd hardly call it "useless."
Death to adding those stupid "font size icons" to change to one of 2-3 predetermined sizes too!