Like I said, you get all resources for free anyway. That's how this game works, resource inside your border=you get it if improved. Straying a way for that is making a good idea complicated and unnatural. So yes, you get the resources for free, and yes that's a big boost sometimes. But remember, building a fort, a road and improving tiles is alot of work for a worker and you're also paying for the traderoute. Plus these hubs will be away from your cities and will probably need at least one millitary unit to fight off barbarians so that's some more maintenance. Besides, if it wouldn't be strong it wouldn't be worth all the effort. Also, if a hub really has that many good resources in that small radius you should probably build a city instead.