1). In my opinion this is primarily an anti spoofing measure. It ensures the raid goes to local players, and someone can't teleport in to every raid and battle and capture this powerful boss.
I think Niantic will be generous with the window they require for you to get the invite and if they do, it will prevent spoofing.
I also think this has the potential to help players find these raids rather than the change method we have now.
2). Rural players will actually have some benefit from this system if there are enough local players to beat the boss. With less gyms it will guarantee that players will have raided in the gym so the only disadvantage is the potential to not have enough people.
3). It adds some exclusivity to this 'mon. Legendary items in video games are supposed to be hard to obtain. Why should this augmented reality game be any different? It should be a challenge to get the best Pokémon in be game and make it very meaningful when you get one.
4). I am a semi-rural player. My community isn't a big city or large urban area but also isn't a small village. At less than 48000 people we fall in a strange grey area of the game. Most days it takes a lot of coordination to do raid battles, and it is rewarding when we win. I have played in even smaller communities and same story, it takes a lot of coordination to make it happen but it's possible.