What really is a defensible space? Well, it can be defined as an area, or zone, around your house that would better protect you from naturally occurring fires, falling trees, or things that could potentially damage your house. The easiest way to think of this would be to split the area into 3 zones.
Zone 1 would be an immediate 15 feet in any direction from the outside of your house, having no flammable plants or even firewood, which should be located a reasonable distance away. Zone 2 would be an extension from zone 1, a total of 75 to 125 feet from the house. Keeping flammable debris like dead leaves, branches, or pine needles must be kept in check. If there are trees or shrubs, it may be good to have them spread out, so as to slow a fire down in case one did occur.
Zone 3 would extend beyond the second zone and to the property limit. A greater amount of vegetation can be left in this area, but should still be kept in check. So if having a defensible space means protecting your house and your property, then it is definitely worth having this space.