In all the talk of graphical downgrades no one seems much preoccupied with 'why?'. Why build something and then proceed to tear it down, piece by piece, in the hope that ever more diminished expectations about the final product won't be severe enough to...
Increase the distance your melee weapons can reach as soon as you can.
You get more for melee kills, so make sure you keep running.
You want to be in front of the scientist at all times, clearing a path.