One thing you will definitely find is that metal as a whole has a very large amount of sub-genres that each seek to bring about a unique sound and style. Sometimes it is about pumping your fist and signing along, and sometimes it is about letting the music take you on a journey or tell you a story. Despite the fact that metal has arguably the most intelligent lyricism out of any other genre, you don't always need words to tell a story. In fact, metal is probably the most complex genre there is; so unfortunately I am going to have to call you out on your false title once again.
One genre that places instrumentation above vocals would be post metal. In post metal the music is more about long winded passages of organic instrumentation than it is about vocals - which does not mean the vocalist can afford to be poor by any means - but it does create less need for a good vocalist. Isis' classic album Panopticon is a good example of this. The vocalist isn't necessarily terrible, but he does not sing much, for the music is more about the holistic journey of organic sound the listener will be taken on. Almost like guitars talking to each other.
Here is that album: https://www.youtube.com/watch?v=xA2AJIiw0p8
Sometimes it is true that vocals are intentionally bad, unintelligable or even held back in the mix to create a unique effect. One such example of this is the Jane Doe album by Converge, which is coincidentally my favourite album of all time. The lyrics are very intelligent, but impossible to understand. This is done intentionally to create emotion and a specific array of effects on the listener. In Jane Doe's genre, the music is shaped around the songwriting, blistering instrumentation and raw emotion of the music - not so much the vocals.
Here is that album, which is not for the faint of heart - https://www.youtube.com/watch?v=zLRYREmJqho
In some genres the vocals are clean completely and quite good in fact. From there they can be taken as clean only or have a little death growling mixed in for added effect. The music and sound mix places more emphasis on the vocals; specifically vocal melody and harmony. Sometimes hearing the lyrics is an essential part of telling the story you want your album to tell. One band who I believe is the epitome of this is Opeth. Listen to their song The Lotus Eater - sometimes metal music can be designed for great vocalists.
You can listen to that song here - https://www.youtube.com/watch?v=XhohQNdSt7g
Anyhow, if you have any other questions feel free to ask me
#1 Amb-ass-ador
