I think the no-helmet thing is officially so they can lead more charismatically.
According to the wiki, certain chapters have special features that can only be used by non-helmeted space marines. The Wolfs can only use their increased sense of smell when not wearing helmets, that dragon space marine chapter and the wolves can bite their opponents, and
most space marines have belcher glands, which only allow them to spit acid when not wearing helmets. And most thought spitting at foes was just for intimidation, bwah ha ha! Armor-piercing spit attack for the win!
I don't like "good as flashlights" when a Lasgun has a low but very real chance of one-shotting an armored Astartes (shoot through neck joint into brain).
From what I can tell, lasgun can penetrate most armor, but they only do pinholes of damage. So they have to hit something critical in order to do any real damage, otherwise it is equivalent to a bee sting (ignoring that bee stings also include poison, to which many are allergic). A Lasgun attack will certainly penetrate the armor of their foes, but they have to hit fuel/arteries/etc.
Presumably, an expert marksman with a lasgun would be quite the menace. Swarms of guardsmen aiming at the same critical areas would also work. It is just that most Guardsman lack the discipline and training to effectively use their default weapon.
...Also, the FAA has condemned laser pointers as being able to take out jumbo jets. So it is really a question of skill and dedication, rather than overtly defective equipment.