head isn't just visual. it maintains carbonation in the beer (freshiness if you will)
and gives a lot to mouthfeel. But, again, it depends on the beer.
Hefeweizen - good 1/4" head
Stout - 1/8" head is fine
Belgian Triple - 2-4" head ideally
head retention is always a must in my opinion. despite the size, i like the head to be there at least half-way through the beer.
some well-carbonated beers, due to adjuncts, cannot maintain head retention. can they still be good beers? sure. but you lose the mouthfeel and the visual aspect.
And although it's not the only thing, visual IS still important. I like my beer to have a good head on it's shoulders