I don't doubt that there is extraterrestrial life. The problem is how to detect it, and, ultimately, communicate. SETI@home is one tool in that discovery process.
The effort to discover ET, will benefit humankind because, like the space program, it will necessitate the development of new technologies. The end result will be better communications, materials, etc. When we find ET, I think that we'll find a kindred spirit in the universe, something akin to finding a long lost relative. I only hope this discovery is made in my lifetime.