Weren't marines speacially trained to fight in hostile and exotic locations? Either way a speacially trained grunt is still a grunt.
I know this isn't the most important aspect of the game, but think of it this way: how many people would be upset if they gave the imperial officer snow trooper armor? This is wrong in exactly the same way in my humble opinion.
Not being paranoid doesn't mean someone isn't trying to kill you... or does it?