One important drawback to use font size in pixels is that it becomes hard to change all web site text font size proportionally after you have finished development.
If you set the absolute font-size in one place - preferably the body element font size - and use Ems in any other size property, you can change the whole site text sizes by just changing the body element font size.
I assume that most users don't know how to scale just texts using their browser zoom capabilities and still web designers want to set a default proportion between texts, other page components and the general layout.
