"If you entrench yourself behind strong fortifications, you compel the enemy seek a solution elsewhere."
- Karl von Clausewitz

Throughout the course of human history, there have been very few cases in which defense has been able to consistently beat offense, and even in these cases, technological advancements have quickly reversed this trend. Even in World War I, which saw some of the bloodiest warfare ever as a result of defense being able to stifle offense initially, eventually both sides developed tactics and technologies that allowed offense to regain the upper hand.

Warhammer 40k is a war game, not a peace game. The rules, and missions are designed in such a way as to force the two armies to interact. By extension, this means that offense is favored, in the design of the game, over defense. And, this means that playing the game with a defensive mindset is playing with a handicap.

There are very few missions that involve simply keeping your forces alive. Missions have objectives, designed to draw your forces into conflict with your opponent's. That isn't to say that presenting a static shooting army is automatically defensive, but at some point, you are going to need to pick up your lines and go and take some objectives.

However, by defense, what I really mean is being reactive. Offense, whether static or mobile, still seeks to take the initiative and impose its will on the opponent. In contrast, defense seeks to be reactive. An example would be a heavy weapon team that deploys behind terrain, without line-of-sight to any enemy units. This is a reactive placement. If the enemy moves to a position where they can be seen, then they can shoot them, otherwise, they cannot, and will have no impact on the battle.

Paying points for units which will have no impact on the battle seems like a good way to lose a battle to me. It is essentially giving your opponent extra points with which to build their army. Since you do not want to do this, you must be active, and not reactive, for taking a reactive stance gives your opponent the opportunity to bypass your reactive, defensive units, concentrating instead on their own objectives.

Even in missions where you are assigned to be the defender, this applies. Consider a bunker assault mission. You can keep your forces in the bunker, waiting for the enemy assault, or you can actively seek out the enemy with some (or all) of your force, hoping to engage them before they are ready. In the first case, you allow your opponent to dictate all the terms of the battle. You give them both time and space in which to maneuver so that they can gain the most advantageous use of terrain. In the latter case, even though you may give up the defensive bonus afforded by the bunker, you gain control over the battle. You are not left at the whim of the attacker, and instead become an active participant in the battle, rather than simply being a target.