Yeah I think we do get used to some things rather than worry about how good they really are. Personal perception is a big thing too, we like what we like, and wont often bother chasing what we don't even if it's only one element of the over all package.
It is interesting to see some of those YT videos where professional singing coaches and the like listen to metal bands and rate the singers. Sure they might be as set up as any other video but to see them listen to a song they supposedly hadn't heard and react to the way the vocalist sings different parts is often quite good. The reaction I've seen several people have upon hearing Kiske sing Keeper Of The Seven Keys is brilliant. But being that good is not required for most bands.
The weird thing I find is that a band like Vio-lence does have a really good sound, especially those first two albums, but while Sean's voice just doesn't cut it for me, there will be other bands where vocalists are no better that do work for me. Another thing is how difference is sometimes good, for instance Blitz has one hell of a distinctive voice, he can go high and low but while he may not be the most technically proficient singer there is no way Overkill would be Overkill without him. I've always like Zetro's voice too, I know some people think he's the worst part of Exodus, but Zetro to me is so distinctive that he adds something to Exodus other bands don't have. (obviously Balloff was better than that but we can't have Paul back).
However I think the most obvious sign that we all perceive things differently is the number of people who hated on Mustaine for changing his singing style around RIP, then changing again around Countdown. We all know he wasn't the greatest singer, still isn't, but for many they claimed he got worse in the same 5 year period where he was actually taking lessons from the same voice coach as Axl Rose. Personally I didn't think his changes were good or bad, but at the time some people were polarised on their decisions about how bad the change was, yet it was a change recommended to him by a 'professional'. It was also a recommendation to make his voice last longer apparently, but any change was always going to divide some people.