I think Ken's point is that with metal in particular, visuals are an important aspect. Look at some of the greatest albums of all time in the metal genre and almost without exception there is something about the artwork that strikes you and stays with you. The more a band focuses on the package as a whole, the more it becomes a larger, more important thing than just the sounds that come out. Or rather, when you take away the art, booklet, etc the music looses something important and is degraded to 1's and 0's. Why rob yourself of half the picture?
I'm not going to say I've never downloaded an album and just couldn't enjoy it because i didn't have artwork and lyrics. It's definitely possible, but I've also never encountered an album that I enjoyed LESS because I had the artwork, pictures, and booklet to look through.
my 2 cents on the topic at hand (from someone who downloads a LOT of music) is that it's always worth buying to show support. Any time people can put some of their hard earned money towards something they enjoy will invariably sustain that channel of entertainment somehow, whether it's allowing the label to continue to release albums, the artist to have a bit of financial backing, or any of that. I honestly feel it's up to each individual to choose how they want to support their favorite bands, but its important that everyone do SOMETHING or else it's impossible to continue. Whether you buy a shirt, buy a cd, buy an LP, buy a bootleg mexican lighter and back patch, tell a few people about a band who then become fans and spend their money, go to a show and headbang.... all of these things help. If a cd is no longer worth it to you, buy a shirt or something!
Point is, while physical media may be on the brink of extinction for some of us here, there's ALWAYS ways to support and excuses are never acceptable