It's interesting because this debate carried on last year, but mostly within the circles of Prog vs Power. Indeed, most power fans, for the most part, would be happy with 10 power metal bands and likewise with Prog fans.
The argument against being; how would you 'learn' about the other side of the fence otherwise? Well, the argument can be extended to progressive or power metal beyond their boundaries right?
That's where bands like Opeth come in.
And where do you draw the line on death vocals? Is it when the entire song is made up of death vocals, or just a few lines here or there? Opeth mix it up, and they are certainly considered progressive. Elvenking use very few death vocals, and they are power. Children of Bodom only use death vocals, but they are power metal too.
There is no winning argument. That's the beauty of being open minded and accepting anything Glenn puts on the roster. Note that he has tried Opeth before, and has been quite vocal about liking Soilwork.