And knowing that life like ours has already developed on a planet like ours, it makes sense that there will be others like it out there, so we might as well look for them first.
I'm not saying that ET life can't be very different to ours, or develop in conditions radically different to ours- but it makes sense to start searching at the most likely place for the most likely thing, no?
It may be the most likely to be easily recognized but not necessarily the most prevalent. You have two variables to balance: (a) likelihood that the planet you're looking at contains life and (b) likelihood that you'll know it when you see it.
If you can increase (a) significantly then it may not matter if (b) is very low or not