Yeah that's probably it. It subsitutes one of the six "necessary" chemical elements (Phosphorus) for Arsenic... which basically means we can't possibly say an enrivonment isn't suitable for life, because a shadow of doubt has been cast over our knowledge about what exactly is needed for life to exist.
NASA doesn't know buttkiss. You have to go at least to the Department of Defense to get those answers. Not that they would give them to do. That information is kept on a need to know basis, and, frankly, you don't need to know.
I think scientists are a bit ignorant to just assume that life 'needs' this. We know nothing of the universe, you can't just claim these things for sure! And these little buggy things prove that
