Having a cover of 2-3 riflemen you can quite effectively move into the vents above the Alien Base, Reac and turret under it will allow you to build the rest of the base without any serious cover from teammates. Base can provide you with 1 armory, 1 DC (or replaced with 2 turrets), 2 nodes, 3-5 turrets, 1 medi. An adv goon from below can snipe the base only if he jumps up, at this stage it is plain easy to take over it with a psaw or anything else. Adv maras are usually supressed by the humans trying to get out of the vents. Its also quiet close to the A base, if you manage to override the aliens defences below your base. Usually it ends up as a cage for humans, but if your team has 1-2 good players, they will be able to sneak out, while whole alien team camps 3 exits from the vents.