Critiques of SETI and Insights from Ask a Mathematician, Ask a Physicist

Is the search for extraterrestrial intelligence, or SETI, solid science, pseudoscience, or something else, as Massimo argues in his book "Nonsense on Stilts"? What are the theoretical foundations and empirical evidence that justify a multi-decade research program, and what are its chances of succeeding? Have we learned anything thanks to SETI? Also, if the universe is infinite, what problems does this pose for utilitarian ethics?