In a nutshell: bad metrics and perverse incentives for engineers. If the only metrics that would move from such a change are ad impressions (these are ads after

In a nutshell: bad metrics and perverse incentives for engineers. If the only metrics that would move from such a change are ad impressions (these are ads after all) then either nobody will make that change or somebody else will revert it to make Number Go Up again. The metrics that are missing can be as damaging as the ones that exist.