Well William's notorious for not helping his wife. He often seems to leave her stranded looking abandoned and helpless, be it in the middle of parade grounds or in a railway tunnel. He doesn't seem very considerate to me.
In the case of Elizabeth and Philip I think that it could be a case of mutual support going down those stairs. He's a good bit older than her - and she's no spring chicken - and they are both looking increasingly frail. In Edward's case, I think his support for Sophie is just what one would expect from the happily married couple I believe they are.