...should vary, depending on what the guard is armed with.

If they have no gun, or just the pipe pistol they spawned with, they're just a spotter and are only worth 2 points, like a spotlight.

If you give them a missle launcher, they're worth as much as a missle turret. Maybe more, as they can move around, and might be harder to kill if you gave 'em decent armor.

Also, the defence value of guard posts should be based on the number of guardsmen, not the number of posts. A single guard can only man one post at a time.

Equipping settlers with better armor and weapons does increase the defense of your settlement, it just doesn't increase the numerical value of your defense. It's not too terribly difficult to get 300+ defense anyway though. That's going to take care of basically anything you encounter.

Think of it like the spotlights: those obviously aren't gunning down super mutants, but they're illuminating the area and highlighting possible intruders. A weak guardpost is the same way: they might not be a gauss-wielding flesh-turret, but they're keeping an eye out and possibly giving the settlement early warning to possible threats. That alone is worth a few points of defense.

...which is why I said a weakly-armed guard should be worth the same as a spolight.

On the other hand, if you avoid turrets and just arm your guards to the teeth, you will get more attacks which gives you the joy of seeing those guards in action...

But it's just so hilarious when a BoS vertibird decides to fly over my anti-fly zones.