system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40,000: Space Marine] Why they don't use the helmets?

Seems really careless for a group so well prepared and dogmatic.

Edit: I'm talking specifically about the PC game.
gpt
In addition to what others have said, Space Marines have genetically modified organs which give them almost supernatural capabilities.  Could be that the planet they are on has an atmosphere that Space Marines can breathe in without their helmets because of their genetically super lungs.  

Sgt.'s and other commanders will also occasionally remove their helmets as a sign of confidence and brazenness (removing protection in the midst of battle) in order to inspire their soldiers.