What a load of rubbish, death metal vocals vary quite a lot and are just as much an expressive part of the music as clean vocals. And this hasn't changed at all in the evolution of more extreme forms of metal over the years. I've never considered them 'shock value', simply an appropriate accompaniment to the low-tone and dark sounding music in the background. Chuck Shuldiner was a great vocalist, but there are plenty of full on growlers that inject plenty of power and delivery into their vocals. David Vincent? Frank Mullen? To name a couple of obvious ones.
I will say though, that 'slam' death metal vocals are just awful, I can't stand them. Shame because the music is often quite good.