Base defenses

[Marrik]

Is there any reason to build them if I am low on space? In vanilla, if you didn't have enough base firepower to destroy the attacking UFO, it was the same as if you had no defenses. Is that how it works in this mod too?

[Mrvex]

Any defences reduce the number of attackers, so if nothing else, it makes the defense slightly easier for you.
Though if you build your base competently, it might be just better to have more enemy bodies and loot in the end, especially if they get thrown in one or two large corridors where they all get mass molested with rockets and miniguns.