it could quite possibly be true that resist is more important than deathblow. however, the major difference is that we're talking about periods of length. people, when fighting ragers, know resist is up. it's expected. they won't see a change in how much dam output they're putting on the rager. it would just be like fighting a stoneskinned warrior or something.
on the other hand, deathblow is unpredictable. it actually changes the course of a fight, as seen in many logs. even if you're near perfect health and the rager is writhing, they can still win from just two or three deathblows sudddenly in the last two or three rounds of a fight, and then you're screwed. i think people are more against the fact that it has the ability to suddenly let a rager win a fight that they had been losing consistently.