I believe that we would we see greater productivity for those 2-3 hours if people were allowed to choose what to do with their time rather than being compelled to spend it at the airport. 

Scoresby

econ

$83 Billion Wasted: Showing Up At The Airport 3 Hours Before Your Flight

They are saying 83 billions lost. I am saying more than double won through the in-airport spend by all those people waiting 2-3 hours. Plus you have to consider that there are airports where 2h-3h are barely enough to check-in, pass through security and get to the gate. So no hope to have this rule taken out.  :)