While discussions like this are valuable, I think they can be counterproductive.
Although it is very important to have "out of the box" thinking when it comes to the search for extraterrestrial life (whether that life be single-celled organisms hidden deep inside Mars' crust or highly intelligent alien civilizations), using life as we know it as a template is no bad thing. After all, we are the only lifeforms we know of in the entire Universe, why would we ever consider Earth "barely habitable"?
How life is sparked remains one of the biggest questions hanging over modern science and the search for extraterrestrial life is an offshoot from that. Perhaps Earth, in its barely habitable state, is actually the perfect environment for life to gain a foothold?
Lacking evidence to the contrary, seeking out second-Earths probably isn't such a bad idea.