As a Tau player, I'm familiar with their unique strengths and weaknesses, namely shooting and close combat. Having played against marines several times I had come to realize they were better than my Tau in close combat, but fell short in shooting. Watching games of marines fighting the Tyranid menace, I saw how the marines were able to outshoot the bugs, but couldn't stand up in close combat. How could an army that was being out classed in one way or another possibly be able to be worth while, I thought.
That's when it hit me. When I was in a shootout with marines, I was generally better, but they held their own. When the marines tangoed with the bugs, the bugs were better, but the marines were able to hold their own again. The marines weren't close combat specialists, nor were they ranged specialists; nor were they incompetent shooting at an opponent or when fisticuffs were called for. The marines did not have any overwhelming superiority, but they have no crippling weakness either.
The strength of marines, in my opinion, is their ability to adapt to any situation and be able to fight their way out. The standard battle brother is able to be used in a variety of different roles, but choosing the role for the squad requires the commander to consider the situation carefully. Sometimes the choice is clear and simple; hit the shooty things and shoot the hitty things. The ability to do either well, not necessarily excellently, but well is the strength of a marine and a quality that can forgive tactical errors by a commander.
Strive always to make the best choice given the situation and leave the rest to the dice.