Much like the title says, the "bro code" system is a cool way to get bonus exp for playing with "Bro's".
Except that most of us have friends who already play the game. And they can't use our bro code. So we can't be Bro's. Those of use who have friends with existing accounts may even form parties and play together a whole bunch - but sadly we can't enjoy a bonus because they had an account created before a certain date.
My suggestion? Add the 10% bonus to playing with someone on your friends list also. That way you can enjoy your Bro's that were your Bro's before the code of Bro's was established. Proto-Bro's if you will.