Well Forts increase you defense by a percentage right? So if you are a heavy attacker you have a low defense and so the increase in defense is going to be low.
For example lets presume you have 1000 acres and are running a standard 40 DPA.
10% forts will probably depending on BE, increase DME by around 10%, giving you 44DPA.
Is 4 DPA going to deter an attacker really? Not having guard stations is more likely to do the opposite, attackers will see you as a max gain target.
But that is not why they are 100% useless. They are 100% useless because even if they do deter someone from attacking you (which is very unlikely), they are not just going to give up attacking, they will attack one of your KD mates and so your forts may have saved you from a hit, but on the scale of things nothings changed; your KD has still taken the same amount of hits.
Guard stations will reduce the amount of acres taken from your KD overall.
If you are a heavy attacker (depending on personality and race) you need Rax, GS, Hospitals and TG. As a human shepherd i would make use of all 4 of these buildings.
Edit. My Human Shep war build:
15% TG
15% Rax
15% GS
15% Hospitals
10% Armories (Or theif dens depending on your TPA)
10% banks
5% farms
8% guilds
4% towers
3% dungeons