(this is an offshoot of the discussion in the 'practice squad increase' thread)
Here's the rationale behind gameday inactives...
Let's say on a given day Team X has two players injured and unable to play, while their opponent Team Y has seven guys out.
Under those conditions, Team X could be said to have a competitive advantage because with more healthy players available they can go deeper into their bench than Team Y potentially could.
Therefore, the NFL has required all teams t