I actually like to keep a burrow in almost all my bases just for base defense. The floor plan is open and is a spawn spot for 2x2 units like Reapers/Mutie Reapers and tanks. Of course its also needed for Werewolves, Dogs, and Parrots. I usually have it close to a hangar or other access point. I also has a completely blocked Air Vent top level, which should be taken into consideration when planning base defense layouts. It also is a stores spawn location so extra ammo, weapons, etc can be found there often. For this reason take care with explosives in the facility.