Well the thread is called best metal singer, and harsh vocals aren't singing, so it makes sense that many are voting that way, even though the OP touches upon growls. Plus that's one part of some sub-genres of recent metal, covering only part of metal history. It's in no way a prerequisite to be considered the best metal vocalist. I think it's a bit narrow minded to consider it that vital.
And being the best vocalist isn't about counting the number of vocal styles you can muster, it's about how good you are at it, so it has nothing to do with "voting with your heart", it's voting with your ears instead of voting with a checklist. Being a jack of all trades isn't something to aim for.
Most importantly to me, there are very few metal singers who do true harsh vocals that I'd call great singers to begin with. I listen to a lot more metal bands with straight up singers instead of growlers, but in my limited experience Christian Älvestam from Scar Symmetry is the only one I've heard worth mentioning. In the same way that some of you might not consider someone who can't growl, I wouldn't consider someone who isn't a great singer either. And I think growling is too subjective a technique to rank, even though there are elements such as depth and range that come into play.
To play the devil's (growlie's) advocate:
1. Growls totally are singing, just a different technique. Most people agree that if you can't sing a note properly you can't growl it either. Regular ole harsh vocals are a bit different, but also take a lot of skills one could only develop by singing (timing, feel, fit, emoting, etc). Growls appear in or dominate
all genres of metal except heavy, thrash, power and glam, and they cover the truly innovative part of metal from 1990 onwards, when it was thriving in the underground. So it seems that nowadays, if you're starting a metal band and you need a vocalist, there's about 70% chance that you'll need someone that also growls.
2. Like I said previously, growls are such a huge part of metal that being able to growl isn't being a jack of all trades, it's just something that goes with the territory.
3. No one really said anything about considering people who are good growlers but poor singers, it's just that dismissing growling as an important tool in a metal vocalist's arsenal dismisses most of metal. I am the first to consider good old Brucie and Ronnie to be the best metal singers of all time, but their era wasn't the only trailblazing era of metal, and there's a lot of other standards metal singers need to live up to.
And yeah, most of my choices would not be able to growl, but I get Shadow Ninja's sentiment.