I'm currently unconvinced either way on this matter. However, enough arguments have been raised that I think this is worth the time of every reader to think a good deal about.
http://nothingismere.com/2014/11/12/inhuman-altruism-inferential-gap-or-motivational-gap/
Aside from painting "LessWrong types" in really broad, unflattering strokes, I thought the author made several good points. Note though that I am a ~15 year vegetarian (and sometime vegan) myself and I definitely identify with his argument, so there's the opportunity for subjective validation to creep in. I also find many perference-utlitarian viewpoints persuasive, though I wouldn't yet identify as one.
I think the 20% thing and the 1-in-20 thing were just hypothetical, so we shouldn't get too hung up on them; I think his case is just as strong without any numbers. There is some uncertainty about the continuum of animal cognition and how it relates to their capacity to suffer.
My own personal voice-inside-my-head reasons for vegetarianism can be summarized as follows: "I am an animal, but a unique kind of animal who can understand what it means to feel pain and to die and who doesn't want that to happen to himself or to any other animals. My unique kind of animal can also live a happy, healthy life at very little personal expense without causing other animals to feel pain or to die." Thus, Rob's first 4 premises (particularly 2 and 3) resonated with me.
I don't believe other animals, even other mammals, have anything like human consciousness. Nor do I believe they should be accorded human rights. But I know that at the end of the day, biologically I am a mammal; if you're warm-blooded and you've got hair and a neocortex, then I'm really going to avoid hurting/killing you. If you have a spine and a pulse, I'm giving you the benefit of the doubt.
Having a small uncertainty about animal suffering and then saying that because of the large number of animals we eat, even a small uncertainty is enough to make eating animals bad, is a variation on Pascal's Mugging.