Is it true that there's basically no use to building fences/walls/etc. around the perimeter of your settlements? I had already built castle-like walls around Abernathy Farm and County Crossing, and was probably going to do the same to The Slog since it seems to be constantly getting bum-rushed by a dozen higher level Super Mutants at a time. I've read that during such attacks, the game will just spawn things wherever, including right in the center of the settlement.
Yes and no. In general, you shouldn't see Gate/Fence/Walls as a way to exclude enemies. You should see it as a way to tunnel enemies towards the lovely embrace of your heavy gun turrets.
Having said that, some settlements like Red Rocket you can gate very well since enemies always come from the front of the area. So if you gate it and you put turrets on the top, they'll shred the Super Mutants to oblivion. Having a gate also prevents them from being able to explode their idiotic brain and destroy your crops before they get shredded by your turrets.
Also, be careful that gating your settlement often means that your provisioners (supply line) can't reach your workbench. In most cases this doesn't seem to be a problem but I've experienced situations in Sanctuary where I wonder if I make things difficult for them.
Last but not least, no gate can keep Synth Infiltrators from pretending to be one of the settlers then attack from inside. Obviously said infiltrators will be immediately disposed by the other settlers or your turrets but they usually brings with them a squad of Synth troops.