Why would the impossibility of defining consciousness suggest that it's not a big deal?

Because it's completely dwarfed by florpedness. I can't define that either, but I just know it's more important.

Less facetiously: A debate of the importance of something needs a shared understanding of what is being debated. Without that, any discussion is merely people shouting that their belief is the right one, and the others are the heathens/idiots - because there isn't even agreement on what is debated.

I'm not asking for a definition of consciousness or an argument for whether it's here or there. I'm asking why the heuristic of "if X is undefinable, then X probably doesn't matter" is a reasonable heuristic.