They're a convenience. I like convenient things, it makes things a bit easier.
I don't get upset when convenient things aren't available, although I do have a little grump to myself about it. But then I carry on, because I can still achieve the same thing, even if it's not as straightforward as it was with the convenience.
Having a moan is one thing, taking offensive action because you didn't get to have the convenient thing this time around because someone else got there first is a bit entitled.
Yes they are lovely to have, P&C spaces, but they aren't there out of necessity and everyone would mostly cope if they weren't available and every one was filled with people using them correctly (parents and children of suitable age). We'd still cope.
So I don't agree with challenging people in P&C spaces. It's first come, first served, and sometimes you get one, sometimes you don't. Thinking you have more right to the space than someone else is pretty entitled.
Parking in a P&C space when you really don't need it, is a bit inconsiderate, but often you may not be able to see the reason why someone needed the space, so I'd rather have a "expect the best, plan or be prepared for the worst" attitude towards people and think to myself that perhaps today they needed it more than I did, and I did get here second, so nevermind.
It doesn't need to be this big issue, because you can be sure about one thing, the people parked inconsiderately are investing as much emotion into this matter as you are, and is it really worth the stress?