The rule is there to mitigate the impact that minor injuries have on individual games. If Team A has 6 players that are too hurt to play a football game, but not hurt enough to go on IR, and Team B has no players like that, Team A is now fielding 47 players against 53. Yes, not every player plays a huge role, but it's still more options and a competitive advantage for Team B.
Giving each team a buffer of 6 or so players means that, unless there's a HUGE rash of injuries in that specific sweetspot, both teams are on equal terms as far as available bodies in a game.
I'm all for expanding the rosters as a result of this rule change, or at least making the IR and PS rules from last year permanent (or a variation of them at least) to help de-incentivize teams from playing injured players, but there should still be a difference between total roster count and gameday roster count.