-probably not Valve.. no real "vested interest" there.
This is probably one of those things that will be forgotten again and again, until it isn't.
Eye-tracking will be available long before this is developed for reliable results - because it's directly useful for limiting several aspects of hardware requirements. And you can be sure that add-tracking at some point will integrate eye-tracking.
The "kicker" to this is when they start altering the adds (based on literal views) and "seeing" your mental response in real-time, and then craft the adds to specifically alter your brain chemistry: to give you a high that ensures you'll hit "buy". I wonder what kind of fines the EU will hand out for that?