You only need to upgrade all items in one guild for all heroes in all that type's guild to get the upgrades. So... a fully upgraded warriors guild will give upgrades to the warriors in other guilds. The only points to buying upgrades in all the guilds is in case one guild is destroyed you have a backup, and/or so that the warriors don't have to travel to that one guild to get the upgrade. They can travel to any of the three instead.