IMO the most you notice it is with the zombies. You realize in S2 he's gone once the zombies are just dumb and slow. Imagine if they stuck with semi-smart, semi-fast zombies. We wouldn't be at the stupid we are now where zombies have to be stealth, hovering above ground, completely silent, teleporting and multiplying like the Matrix to be a danger. Or even if they just made it so that you have to knife/shoot them at a specific point. Only the brainstem is active according to Jenner, yet zombies are always taken out by stabbing/shooting the regular brain which the zombies don't even use.