True enough that we won't be able to defend a pos against a full scale attack.
The question is how often do those take place?
POS warfare is hated by almost everyone, so the benefit to do it needs to be large enough to risk anything. I seriously doubt anyone will do it for a chrom or cad moon defended by a large tower and if they do it is going to be forces that can field multiple dreads which means even a alliance with nerf and that alliance is not likely to be of much use.
Also, true that we are small, but we have altitis.
We look much larger than we are.
I'm not against cooperating with them and all but I'm hesitant to join another alliance, it might be better to just have a friendship and cooperation pact so to speak that if they need help we help and vice versa.