Defense in depth expands killing potential by prolonging ranged exposure while at the same time reducing the threat potential of concentrated attacks and their splash affects upon your defences.
1. The longer the opportunity your guns have to fire at a target, the greater the damage they will deal.
2. Dispersed defences take much longer to kill since the enemy must move between point defences, and in fact a lone point defence will often be ignored by an enemy player. The travel time between the defences will increase the weapons exposure time.
3. If all of your defences are concentrated they are very vulnerable to overwhelming concentrated attacks. However, a horde of t1 bombers will accomplish little if your defences are dispersed.
