I think the crux of the argument is: "how difficult is too difficult"?
See, some people die, lose half an hour and go "Awww man, this game sucks what a waste of my time." While some people die, lose half an hour and go "Well, I better not die again."
One may see the sparse use of checkpoints (and, btw, they ARE there. You may not respawn where you die, but you CAN open shortcuts so you die you don't have to go through the whole stage again) as a deterrement. As making the game too hard for the sake of making the game too hard. One may also see it as an incentive, to get better at the game.
Demon's Souls is scary. It's quiet, moody, desolate. You're alone and death stalks at every corner. But you're only scared if that death MEANS something.
Every encounter can lead to your death. But every encounter can also, just as easily, lead to your taking 0 damage. If you respawn five minutes earlier, death becomes MEANINGLESS. The game loses its fear, cuz all you have to do is rush headlong and use trial and error to win.
But if you respawn a HALF hour ago, you don't WANT to depend on trial and error, because that's a waste of time. The game then forces you into being careful. Survival through paranoia. You block at every turn, dodge roll at the slightless noises, so on so forth.
Demon's Souls challenge is intrisically tied into its world. Its ambiance, its immersion, is all tied into its challenge. Make the game easier and you lose all that makes the game good...