If you were to buy juggernaut, kill the monkeys and get a free perk. Then the next time you got monkeys you would camp juggs again as that is the perk you want to keep, then surely you would lose the other perk as you are protecting juggernaut and thus you would not get a free perk everytime, only the first time. Or if you were ninja and could protect all of the perk machine's at once..
Make sense?
Anyway, the map is actually one of my favourites, however I have not yet tried Call of the Dead or Shangri La. It is awesome split screen. Love Gersch Machines, flopper, landers. And solo is easy if you 'trail' or 'trainwreck' you can pretty much survive forever, however it is quite boring and can get a bit tedious.
GET ASCENTION
You can get a free perk each time if you leave the doors closed to what perks you got from monkeys.