No, you don't HAVE to include the site name, but if you choose to and submit it as your logo - according to my way of thinking - it becomes part of the logo, not just some unnecessary text crammed in there for the sake of cramming it in there. (Some people do this, though, which is maybe why they consider that text disposable). IMO, if the words don't work, then don't put them in there to begin with.
Corran consider the site name in the logo an optional component, however, one that can be removed if it's too small to suit its purpose. This is actually beneficial, as I was trying to say, because I've seen some logos in the past that were really nicely done except for the fact that the text wound up being so itty bitty that there was no way the logo - as a whole - would work and I felt obligated to vote for a logo that DID work, instead. I really don't remember if - in the voting guidelines - it was mentioned that small text shouldn't be a factor in our decisions.
I'm keeping it in mind, this year.