my siblings do this. it mainly worked/adapted to this because:
our parents gall each other with the first name
it was annoiying for them to differentiate our 5 different voices to all the other childs at the playground or parades and other public events where a bunch of kids where present
Edit: and i like the system more because i was very annoyed that i had to call other parents: {friends} mom, can i have a drink? because never heard their name. Thatvwas already infurating during kindergarden times