In one word. It would completely disrupt the data mining business.
They only know when you click on something nowadays. With this tech it would completely change the data mining business and the info it can extract on the levels of engagements.
I predict there will be a system in place to prevent the cam being used at all times though.
Google can come up with something on Youtube streams where the streamer is using the front camera or maybe revamp GoogleTalk with ads, bottom line is giving the user a reason to use the front camera and then capture the gaze