Originally Posted by DArtagnan 
No, that's not what I'm saying.
I think there should be one difficulty level - the correct one.
Anything else will require metagaming, and it will compromise the vision (if there is one, beyond simply making money).
OK consider this: You've got a game were there is a fight with a monster that the developer has 'envisioned' to be hard enough to take you half an hour, about four or five reloads and a dozen or so curses towards him and him family to beat it. Now say that I'm pretty bad at it but I am heavily invested, as you said, so I persist. Eventually it takes me two hours, twenty reloads and two broken keyboards to beat it - 3 times more than the dev intended - the intended pacing is ruined and therefore the vision is compromised…
Allowing me to reduce difficulty at that point is as I see it the best way to maintain that vision. To generalize: games are by definition an interactive medium, and as such, for a creator's vision to be successfully communicated, it is essential to take the particularities of his potential audience into account, more so than in any other non-interactive medium (otherwise the result will be what the art connoisseurs very colorfully refer to as 'masturbation')
Of course one could argue that if I'm so bad at it I should simply stop playing… but why? does that game has something to offer that I shouldn't (or wouldn't be able) to receive unless I was skilled enough to play through it? There might be such games - Wizardry 4 comes to mind, which I have not played, among other reasons because it was obviously a game intended exclusively for the series' most dedicated fans and noone else was going to 'get it' even if they somehow cheated their way through.