Doubtless by the time this posts this will have crossed with 74 other people or Mumsnet will have had to delete for disablism. (Incidentally, epic win for that being in the guidelines...)
It can be tricky. I am a cardcarrying cripple disabled, but, like lots of other people with disabilities, have (hugenormous) variations in functional ability. If I am able to cope with using the ladies I will do so because I am only too well aware of how frustrating it is to have to wait for someone[s] who think it's fine to use the disabled loo because they don't fancy the queue for the ladies. I also once ended up having a rant (whilst waving my Blue Badge in a slightly Leninesque fashion, it must have looked awesome...) at two olderish (prolly in their 60s) women who were pass-agg-ing at me in the queue (yes, queue!) for the disabled loo at Sadler's Wells. I'd got to the loo at the same time as a guy in his chair (with his carer) who I sent in ahead of me as I knew I could hold & he was looking a bit desperate. He was in there a while & they were sniping away at me. Eventually one of them said "you do know this is the disabled toilet, don't you?" and I'm afraid I snapped; and, grabbing my Blue Badge from my rucksack to brandish at them, went into a bit of a tirade about how yes I DID know and I really DID need to use it because I needed the grab rails & easy to use taps & sorry but I was going to be a while because my dislocating fingers were going to make getting necessary clothing off/on tricky and I did hope that THEIR disabilities weren't going to cause them to become incontinent while they waited. I figured I was fairly safe given their queue-ramblings had revealed that one of them had "a poorly foot" & the other was with her for moral support. They looked a bit ashamed and muttered about how it wasn't OBVIOUS I was disabled and if they'd KNOWN (etc...)
Have only once been waved to the front of a queue for a disabled loo. It was at G.UK's Centenary Launch Party at Crystal Palace & several Guiders INSISTED I had to go in & use the (delightful) disabled portaloo first. I actually felt really guilty because the Brownie waiting was doing the Need A Wee dance & she got shouted at by a cross!Guider for even thinking of using the disabled loo. I was really rather 
I have, however, had people attempt to shove in front of me to get into the disabled loo because there's a big queue in the ladies & their child(ren) can't wait. Uh-huh...
The biggest issue is the fact that in so many places the baby-change facilities are in the disabled loo. As (at least one) someone else has said, is the thing not for Mumsnet to use its powers for good & get a campaign going to improve toilet facilities for [people with] small children?
Of course, there will also be wee people in buggies who are disabled & need to use the disabled loo, but you'd not be able to tell that Just By Looking. Like my niecelet, fr'example. And obviously there are parents with disabilities who'll need to use the disabled loo, too. I just have a sneaking suspicion that's not generally the case...
I think there's nothing wrong with diving into a disabled loo in an emergency (e.g. about to be sick & other loos Too Far To Make It) and if that's the only place there's a babychange of course you'll have to use it, but I think doing is As A Matter Of Course is Not On.