People with some disabilites have the ability to wait to use the loo and just need the extra space. For others, waiting is absolutely not an option, as is the case with urge incontinence, unstable bladder and irritable bowel syndrome or disease.
A normal bladder doesn't send the need to urinate message til it's roughly 300-400ml full, and even then, the person can usually wait at least 5 minutes to use the loo, typically longer although it is uncomfortable. But mine can be as little as 10ml full and i suddenly feel like im on the verge of wetting myself full to bursting. You get a minute or two to get to a toilet, or it's coming, ready or not. There is no holding it, you physically cannot wait in a queue.
When we need to go we need to go NOW, at home we don't get to wait 10 minutes til the next ad break during a tv show, or the last 20 minutes of a film, we can't handle "it's just a 5 minute drive home". And it isn't like a lot of cases of stress incontinence where it's a dribble, it is your entire bladder contents, the flood gates open and you have no control.
IBS (and of course other bowel disorders like IBD) can be similar, you get very sudden cases of diarhoea with zero warning, it can be triggered by something you've eaten, by stress, or sometimes no cause you can pinpoint, and you literally get that awful pain in your stomach with the bubbling and that need to go NOW, again you cannot hold it to stand in a queue.
Both of these are awful illnesses and absolutely should be considered disabilities that warrant the use of disabled toilet facilities, should there be a queue for regular toilets.
People who suffer from these illnesses already have to plan very carefully where they go, what time of day to not hit rush hour traffic, that there is always a toilet within a distance they can make it when the urge hits. Some (like myself) can't go out if you have to walk to get where you're going as there's no toilets on the way, and can't use public transport as you can't wait for a bus/taxi or need a direct journey home. It's definitely disabling, and sufferers deserve to keep their dignity by using disabled toilets when needed instead of literally wetting or soiling themselves stood in regular toilet queues.
If you've ever suffered food poisoning or a moderate to severe UTI and so experienced what it is like to have only limited control of your bowel and bladder, and had the very real risk of having an accident just getting to the loo on time from your sofa when the sudden urge hits you, then imagine that being your life 24/7, that's what people with these illnesses suffer from.
Fact is, there aren't enough disabled toilets. One token disabled toilet per toilet facilities in a shopping centre etc is inadequate. It often takes physically and in some cases (like myself) mentally disabled people longer to use the toilet than an average able bodied person. Whilst there may be much smaller numbers of people requiring access to a disabled toilet than a regular one, it always runs the risk of someone who can't wait, coming along and the disabled toilet is occupied with somebody else who is going to take another 10 minutes.
Baby changing facilities need to be stand alone and not lumped in to the disabled toilet, it's one more way a disabled toilet can be occupied by someone not actually using it blocking someone with a genuine need.
It is infuriating when people not disabled use them for convenience, i've known parents with several children (none disabled) go in as a family instead of all waiting in the regular toilet queues, and women with prams using them purely for the space. If provision is needed for people out alone with pushchairs to have larger cubicles, then write to local establishments, council ETC and ask for them, don't use the disabled toilet and possibly be the cause of someone having an accident.
As to the person who used it to breast feed, that was just ridiculous. If you don't feel comfortable breast feeding in public you could try to express or take formula as a last resort if you're going somewhere without baby change/breast feeding facilities and you're uncomfortable doing so in public.