I never beat most of the NES games I owned. The exception was the Super Mario games where you could skip most of the game by warping. Did this bother me? No. Did this frustrate me? Not much. I didn't have to beat them, I enjoyed just playing them. Sometimes I made it to a new area and I'd feel the adrenalin start flowing.
I don't get people playing through a computer game with an invincibility code - and doing it over and over. I'm no hardcore gamer (I'm on a Mac Mini dammit!), but I grew up with hard games and enjoyed the challenge. I also believe many of today's games are too easy with their infinite lives and checkpoints/respawn points once every minute. I don't get the same adrenaline kicks anymore since I don't need to be afraid of dying.