I view lyrics as infinitely important: they have the power to make a song better or worse. In my opinion an album is beyond just a number of songs put together under one title: it is a piece of art in which every ingredient is not only important in itself, but also for the whole. A bad cover can take the shine off an album even if the music was good. Still, the cover should be good enough for anyone to be able to view it as an individual piece of graphical art. The same goes for lyrics: they SHOULD be good in order to strengthen the unity of the album. I would probably still have Dimmu Borgir's "Enthrone darkness triumphant", which is musically good, if the lyrics were not such complete bull. The album lacks the final touch of magic that could make it something I can consider a 'classic'. How sad it is to notice they've gone even further down with their latter albums! In addition the lyrics should be good even if read as individual pieces. Ike from Babylon Whores writes amazing stuff that is great to read even when not sung in the band's (undeniable marvelous) death rock music.
As for Metalmancpa, I understand your point of the vocals being an instrument alongside others, but I think of it this way: the other instruments have a tone and a melody. The vocals have them as well. But they have a great advantage: they can carry a message far better than anything else and that is a possibility to be used when given a chance. Naturally, the message is a subject in itself to be discussed further, but that is not the case right here and now.
Surely lyrics will not ruin anything by their absence; what degrades the impact of a song is the lack of recognition of the lyrics' worth and hence having them for the sake of having something to sing. The 95 % of all bands that fail to distinguish the necessities from their own abilities should go a cappella or start playing instrumental music. Crap lyricists never seem to understand that, and while they have the right to write whatever they want, it is rather selfish to force the lyrics into the music. So, the point is not that lyrics are an unvaluable part of a piece of art. (Hey, that rhymes!) They do not need to be there, but if they are, they should be given the amount of thought they inherently demand. (Also, a death metal band singing lyrics that say "a hrrhguf phoorgh craagh shahhhh" should sound immensely funny - if reading the lyrics, that is, since most sound like that anyway..)
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N/P: Circle - Pori