The point of milling isn't about how well you can punch or if you can put the man on the floor. It is 30 - 45 seconds of pure aggression, that's all. And what they are looking for is to see how aggressive you can be. They then can use this and turn you into what they want. It's about being able to turn aggression on and off. Which is a valueable thing to be able to do. I personally think it looks great fun and would love to give it a go!
The Royal Marines don't to my knowledge us milling in training as they have their own ways of doing things.
I wouldn't knock it as male bravado though. It is only 30-40 seconds long and as I've already said just designed to show how aggressive you can be, and if after (attempting) to brey your oppo, can you turn it off and be mates.
It has been used by the paras for many years, and as I understand it some SF outfits use it as part of their selection process.