In addition, we have a large sample of water & carbon based life. We would be able to pick up on more subtle clues if we saw another water & carbon based life-form than if we found a life-form based on other raw materials. Being able to pick up on subtle clues is incredibly important for the search for microorganisms within our solar system, or to have any search of life outside or solar system that doesn't involve looking for signs of intelligence (Scanning for patterns in radio waves, looking for dyson sphere construction, etc).