Consciousness sounds like an extremely beneficial evolutionary trait to me. Imagine an "unconscious human" who shares the same body as us but the brain thinks it can't think, thinks it is dead, thinks that nothing it senses is alive or real including itself and its own thoughts. That unconscious human can definitively exist as an organism but it would have a hard time to coexist in a community of humans.
Please don't nitpick the usage of the word "think" because I'm referring to "machine style" thinking. A computer can think without being conscious.
Of course, doing that removes any sense that humans are special or unique, which was darawk's point.
This path can lead to solipsism, but there are many exit points between "I am the only conscious being" and "all living things are conscious."
The problem with qualia, subjective consciousness, and all their near-synonyms is that they're almost impossible to define non-self-referentially. That doesn't mean they don't exist, though - and all human experience is indication that they do.
Well, what would you expect to be different if you weren't? Presumably you'd still behave the same way that you do now (because by definition qualia aren't the externally observable parts of your experience), so none of the experiences that affect your decision-making process can be part of them - so wouldn't existing without qualia feel exactly the same as existing with qualia? In which case, why are you sure that you're not already doing it?
What if you're not actually conscious, you just think you are?
But that only works for an outside observer. I can't be tricked into thinking I'm conscious, because it's recursive: who, then, is getting tricked? I know that I'm conscious because I'm the one experiencing things. You know that you're conscious for the same reason. Neither of us has a good way of being sure that the other one - or anyone else - is, at least not without some deeper philosophical reasoning.
Again, seriously, how can I tell? There are people who can't visualise things in their head, and largely don't realise there's anything strange about them. I can certainly introspect my thoughts, but I can talk about the results of that, so presumably a p-zombie is also able to do that. I don't think there's any part of my experience that doesn't affect my actions, so I don't think this notion of qualia is valid.