I think this is the biggest take-home message. I paid for EBCI for my flight home from Disney. I was assigned A30. The woman who had A29 also had two kids with C boarding passes. She tried to sneak them in with her, and EVERYONE in the A group heard her tell her kids "it's ok, just stay with mom, I paid for my seat so you can come with".
When she got to the gate, the agent wouldn't let her take the kids, stating that entirely too many people had paid to do it the right way, and she was welcome to do family boarding with the youngest, but one of the parents (Dad had a high A boarding position) would have to board later with the children who were over the family boarding age.
Needless to say by the time she was able to board with the kids rolled around, there were no seats left to sit together, and nobody was feeling especially charitable toward her since she had tried to cheat the system.