I wonder whether perhaps it ties into the idea that your house is supposed to help you access your potential; Neville was intimidated by the idea of being in Gryffindor, but the Hat put him there because it could see that he had the potential for great bravery and Gryffindor would help to draw it out of him. Maybe some people don't have natural reserves of bravery or cunning they can learn to unleash, but I can't imagine anyone not even having the potential to work towards a goal. If you're not naturally especially brave or cunning or intelligent, Hufflepuff is the house in which you can learn to excel regardless through effort and perseverance. And then, as you say, there are people like Cedric; he does have qualities that might have seen him accepted into other houses (Gryffindor, for example), but he's in Hufflepuff because his Hufflepuff traits are stronger, rather than by default.

That is a bit of an odd message! Reminds me a bit of Merlin, actually, which very much gave the message 'helping people creates problems; killing people solves them'.