It'll spoil it practically not legally. Because people will claim disability, and bus drivers, restaurant owners, greeters, etc. either won't believe them or won't know the law about it. So you'll end up in an argument with them, possibly getting unfairly removed or not let in to the establishment, if they consistently see people claiming disabilities and doubt it., well no, no card required by law, simply saying your disabled should get you a journey, which is what their policy says, you may , but almost certainly wont have to dispute it with the police. the cards themselves are given out with out any proof of disability being required.
and anyway insisting that the disabled carry cards is its self discriminatory, unless required by law, which it isnt
how will it spoil it for the disabled ? it wont make any difference to them, if lots of people claim to have breathing difficulties
Like I said, legally it won't change anything, but you and other disabled people would have to confront them about it and/or escalate it more often than if a card was involved.