it's not so ABS like Richter von Manthofen thinks
Remember how Jupiter was hit by Comet Shoemaker–Levy 9 ?
some thing like that on Mars could bring a short Mars spring.
with thickening the Mars Atmosphere and kickstart a greenhouse effect years later.
it will take sever decades, even centurys until by Solarwind stripped away the new atmosphere.
So what will give this for Manned Mars flight ?
except that Rober Zubrin goes on holy crusade against US Congress and Senate
The thicker Mars Atmosphere give better landing opportunity with parachutes or Lifting Body type craft
instead like the rocket powered landing of the last Mars Rover
also give the thicker Mars Atmosphere better protection against solar radiation
the gases brought by the Comet would be useful, like Nitrogen or Water vapor.