Not neccesarily that small! 11MDs+1 grav shield is almost impenetrable (12 shots give 50% chance of destroying a battleship, and here you have 22), leaving you with 24 tiles to build; 3 hangars+2xstores+lift+HWD is only 16 tiles
Also, this is actually an equal to 4 Fusion Ball Def + GS, since merely 2+GS yield only about 60% chance of success.
Having said all that, I strongly support the notion to either:
1. defenses inflict casualties on the crew, killing some, wounding some of the invaders even if the ship gets through;
2. bases being attacked by various ships, thus less-than-total defenses make sense;
3. defenses being able to engage enemy ships on geoscape. While actual combat would be kinda not cool, the defenses could (upon inflicting enough 'hits') force smaller UFOs to break away from their flight pattern, thereby preventing pin-pointing of the base location (and disrupting other missions). The interception range could be about 250 nautical miles (and we could explain the inevitable logical questions away with the notion that AA defences being physically not in the base, but in a wide circle around it).
4. defenses having some extra function, like ex.
- military posturing (extra points per month?)
- morale boost (+% capability of all living quarters?)
- deterrent (defense strength in region relatively decreases the attraction of aliens to that region)
- friction factor (all UFOs within 1000 nm of the base are slowed down, % based on defense strength, rule of diminishing returns in force here)
- defense facilities housing stationary weapons helping in base defense mission proper;
- or something else altogether.
