If Mrs John Smith or John Smith Esq. were in a position to offer my child a job they wanted, then I would want said child to know the “proper” way so they could decide whether or not to use it.
I'm not talking about the person offering the job, though. Are you really saying that you'd have no problem if your married daughter introduced herself as Lisa Smith, the employer noticed her wedding ring and asked what her husband's name was, and then gave her a contract, a badge to wear at all times and a sign on her door, all saying 'Mrs John Smith'?
If this would be fine with you, how far would you expect her to kowtow to her treasured employer's 'requirements'? Stiletto heels and full make-up at all times? Make sure you show a bit of leg as 'our older pervy wealthier clients like that'? You can be in charge of keeping the kitchen clean and making all the tea, as it's nice to have a woman's touch?
I just think that some people, however important they are or believe themselves to be, need to accept other people's boundaries and realise that you can buy a person's professional services but you cannot buy the person themselves. Power, wealth and prominence shouldn't be a controlling get-out-of-jail-free card, as a certain prince may have recently discovered.
I realise that this might seem like a huge jump, but it isn't really. Once you start letting another person dictate that you hold your cutlery or your pen in a way that is very uncomfortable for you, just because they believe it to be 'the correct way' and have decided they will take personal offence and/or get upset at your not putting yourself out and jumping to their tune, where do you set your limits?