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Let's assume you lose n hours' worth of XP every time you die. In that case, the average interval between your deaths must be over n hours for you to progress at all. In my opinion, if I don't die at least once per hour, the game I'm playing is too easy. On the other hand, if dying only costs you a little XP or nothing at all, the game can be as difficult as it needs to be, because you'll still be able to progress if it kills you brutally again and again.
Improving your skill at the game will make you die less, of course, but the fact remains that the game you're playing must be designed around player deaths being few and far between.