It's more difficult to read uncapitalised prose. I don't know if this has to do with English not being my mother tongue, but I was taught to expect very specific grammatical behaviour from it, and when someone fails to deliver I get all confused.

That said, capitalisation is not a grammatical rule - it's a stylistic convention which differs from place to place (even the "official" rules on how to capitalise things in an English lit university course are hotly disputed between different universities in different countries). So let's not get too pedantic about it, either.

I think capitalisation is good netiquette - it's hard enough on the eyes to read off a flickering screen as it is, without any added confusion. On the other hand, if someone is too lazy or too artistic to care about following the conventions, it's their loss - people are less likely to read what they write.