YANBU, although I do get why some folk think it's getting all worked up over not much. I agree it's only a car park space and yes it's not that important in the grand scheme of things.
But, in common with many other posters, it is not about my DD staying dry or me being able to park as near to the store as possible, it is about width of space. I don't mind where the spaces are as long as they are wide so I can get DD out without bashing other people's cars and they have trolleys nearby. Quite happy to walk a bit etc - I think it does DD good to experience the weather, rain or shine.
Also, IMO, people who don't care where they park also don't seem to care about other rules and standards eg exceeding the speed limit (especially outside schools), talking on the mobile whilst driving, jumping red lights, parking on zig zags etc etc. For me it's about respecting the society we live in and caring about the impact your actions have on others. People who don't care about others, don't care if they have made a parent's day that little bit harder by taking the last P&C space when they have no child with them.
And they wonder why our kids are growing up with no manners, respect etc....sigh.
Before I became a mum I would never have parked in a P&C space - same as I would never speed past a school at home time (and still wouldn't of course) etc
And I have to admit, that as a company car driver, if I have to park in a standard space and some idiot parks too close to me (especially when it's obvious I have a car seat in the back), I have no qualms about bashing the other car door with mine. A bit of common sense is needed. If I'm out and about I try not to park too close to anyone, as everyone needs space.
And, just an observation, I tend to find that the people
who hog the P&C spaces without children tend to be BMW or 'chelsea tractor' types. Not jealous at all of their cars, I drive a very nice car thanks, but again, IME, people who buy BMWs, 4x4s etc tend to be poor drivers, poor parkers and quite self-obsessed. As I drive 25000+ miles a year, I feel I have some experience in this area.