damage spread actually changes consistent things, though. you have a 50% chance of bodyshot-killing a medic instead of a 100% chance. that's not to say that turning it off would be harmful at all, but it's markably more different than what 90% of the maps over here are made for. it'll make snipers slightly better with the rifles, worse with the arrows, increase the effectiveness of the direct hit, etc etc
the randomness is highly distributed already, to the point where after like 10 shots, statistics and prng-suckiness make its effect not matter outside of "plays" like "the medic took 149 damage! he pops! he wins the round! the sniper cries in a corner and kills himself" stop mattering. also, it slightly changes the falloff (softens the middle-high and middle-low, but not the high, middle, or low) for high-firerate weapons.
now, on shotgun spread, I WOULD say it should be universally disabled, but the problem is that it makes shotguns like 2x at powerful at long range because they stack the center pellet (3x for the FaN) and 1.3x as powerful at mid range due to the actual shape of the pattern (seriously, it doesn't match the random spread at all). valve is never going to fix these problems.