If Security rating doesn't spill over, then what is the point of turning some settlements into a Barracks with like 235 security rating with very little food, water, and industrial production? It feels like a complete waste.
Defense is not shared.
At some point, however, all settlements being useful gets completed boring so I wouldn't call it a complete waste.
If usefulness is a stinging concern, take advantage of the change plot type tool.
Each plot allows you to switch it to another type in place via the asam sensor. This, allows you to customize a City Plan for your own particular needs and to do so without changing the plan's layout or without having to search for an empty patch of space.
But, also, consider the long view.
I'm something of a min-maxer, unfortunately. I can put together all the Northwest settlements in such away that they are all cogs in a well oiled machine. Massive farm surpluses and Industry running at peak performance.
There comes a point if you play a save long enough all this usefulness becomes a problem.
You produce too much crap, your workbenches are overstuffed. And you still have other settlements to take over/build.
There have been games that I've had to raise taxes not because I need the caps, but to lower production output.
And then, after putting this well oiled machine together, I'm always left thinking what now? I've got the rest of commonwealth and Far harbor to build.
There's what 25-30 Settlements? Not all them need to be "useful." Sure, in the beginning of the game, usefulness can be a big concern. But starting mid game and into the late game less so.
How many 4 farms/4 martial/4 Industrials/X Commercials before you say enough already? Let's make something crazy! How about a Recreation town?
I could imagine RR being the tip of the spear protecting Sanctuary and Abernathy and controlling the road from Concord.
Something like this would be fun and useful at a place like Country Crossing, too. Guard the crossroads!
Eventually, if you have a well managed settlement network, you can build and support all sorts of fun things, both useful and not.
