The point is that, while it is obviously important to fulton and build MB, that is not 'the only way to play the game', and it's not the way the game 'wants' you to play, which is what the person was saying. Once you've gone fulton happy on tens of missions and you have an oober MB, then what are you going to do? You don't need to fulton any more. The game isn't going to say 'well now there's no need to fulton people and go non-lethal, so game over!'
More importantly, each mission can pan out in hundreds of different ways, and the game gives you the freedom to replay any mission at any time, usually with different variables (deployment places, etc) and with literally no repercussions. So saying the game wants you to play a certain way is moot. You can play however you like. There's always time to go again and fulton more if you want.