You say that like it's a bad thing.
It is when they dictate to the DM “this is how it should be played because it’s not strictly forbidden by the RAW.” Then get pissy for the rest of the session.
There is this idea floating around these boards and others and games that players have an inalienable right equal to or greater then the DM to run the game, especially when it comes to rules decisions about their PCs. While the DM and the PC have to have a back and forth, it is always a DMocracy.
That’s why I have and always will have a PC building session as session 1. You will roll your abilities right there in front of everyone, and you will go back and forth with the DM and group with your PC concept and backstory (brief) and what your ideas are.
Today of course with email this gets done through that, one of the great benefits of easier
communication.
This makes everything easier down the road, especially when 5 players realize “hey, wait, we are all the same class and/or race. We need to branch into different classes to get all the bases covered.” It’s made even easier when I tell them what general type of enemies you will face.
Everyone can get their PCs started and get an adventure in to clear level 1 or maybe even level 2 after the pizza delivery. After that players can flesh out their backstory and make any adjustments needed.
As a DM myself, and as an argument FOR MC, I will allow just about anything if your backstory and RP is good enough. I don’t care about the MC requirements either, if you have a good story and plan it’s ok by me as long as it isn’t a blatant min/max power grab. Even those are fine with the right sort of player and backstory and concept, but most who propose something along those lines and are adamant about it are poison for the table long term.
Really good backstories and concepts are great and if they require a MC with or without a DM rule bend they should be encouraged. I once had a player play a dwarf fighter/sorcerer (with a 7 wisdom) with a split personality, sometimes he woke up as a dwarf male fighter, sometimes as a dwarf female sorcerer. He would flip personalities in the middle of an adventure or combat in response to something, sometimes the player rolled randomly each round to determine what personality would be in front.
I put in magical traps that would force alignment and/sex changes to PCs, which of course would have no affect on that PC. Any attempt to mind control the PC would result in a confusion type affect, as you can’t control both personalities at once. Everyone had a great time.