I hardly think that the absence of extraterrestrial life insures any kind of great future for humanity. A lifeless void would surely guarantee a healthy supply of real estate and resources. But it would also encourage those dimwits who think there's no real reason to have a space program BECAUSE it's a lifeless void.
If we found life, I think it would promote research and funding, and reignite the interest the general population once had for exploration.
Pretty rocks and flow charts don't inspire Joe Public to do anything but flip the channel.
If we found anything that was alive -- like Europan Sea Monkeys -- at least some people would wake up and have something to occupy their minds instead of Lindsay's or Paris' recent pantiless reemergence from rehab.
We need to know that we're not alone.
The religious zealots need to know it so they'll stop harping about how 'special' humans are. I mean, if God(s) made us in His/Her/Their own image, then who made the Greys?
We need a kick in our complacent primate booties. Even if they're monstrous bloodthirsty creatures from Whatevatopia, or just Tribbles.
We need to know that we don't own it all, and we just might need to push harder to make ourselves better than we already are.