I was associated for years with a SETI project, and yes, you all are right, your chances of finding anything are close to zero.
However it’s because the underlying assumptions are self-serving. Imagine you are a radio astronomer, not getting a lot of attention, losing funds to the Hubble, optical multiple mirror telescopes, and the like. What do you do? Assemble a highly questionable set of assumptions suggesting that any ET would naturally communicate on radio frequencies. How convenient! No doubt you, radio astronomer, have great insight into the minds of alien civilizations probably thousands of years in advance of ours. (Citations: NASA SETI materials; B.S. by Frank Drake about his ever-revised equation.)
Revtim is completely right. There are legitimate research projects which could use the extra CPU horsepower.
Also, although I realize that rhetorically I should keep my central point clear of side issues, running a computer with the CPU at 100% takes far more energy than when it’s idle, and even more than one that’s turned off. These “CPU gifts” are actually fairly expensive.