The 100 soldiery bonus is simply the easiest way to see if someone has or has not cleared a fight before. Sure, some can get carried but it's still a really easy metric and a really easy one to use before the fight even starts.

If you think you're entitled to join a farm party without clearing it, that's hilarious. Go build your own 'farm' PF consisting of only people who claim to have had the boss to 1~5% and then tell me how well you do.