This is certainly a very worthwhile pursuit: the principle of Growth implies being open to all sorts of possibilities ... and the existence of intelligent lifeforms in other star systems, galaxies etc. is certainly a feasible possibility.
The Singularity notion casts SETI in an interesting light. Suppose that, shortly after developing advanced technology, a civilization tends to reach a Singularity and its members then advance their intelligence dramatically. In that case, it might well be that there are very few civilizations in the universe at the stage of advancement where they would be interested to communicate with us.
John Smart has suggested that once intelligences get smart enough, they want to maximize their intelligence via packing their processing units into the smallest possible volume of space (to minimize communication delays that are inevitably proportional to the distance betwen processing units, according to the physics of special relativity). So, he suggests, the smartest minds will become so dense as to become black holes. Maybe the black holes we detect with our radio telescopes are actually superhuman minds.
On the other hand, perhaps superhuman minds have somehow distributed themselves throughout the universe, expressing their thought-processes in terms of the quantum or subquantum vibrations and interactions of particles. In that case there would be alien superhuman mind within us and everything we interact with. Perhaps what we think of as "quantum noise" is actually highly complexly patterned -- but they're patterns we're unable to detect or understand, because they're the thoughts of vastly more advanced intelligences.
Stanislaw Lem's novel Solaris remains the most insightful thing yet written about SETI. It features an alien ocean that is clearly intelligent in some sense -- probably superhumanly so -- yet is so very alien in its mind-patterns that we humans have no hope of communicating with it in any useful way.
Greg Egan's novel Diaspora presents a possible solution to the problem Lem poses: build a series of minds forming a bridge between ourselves and the alien. In the Solaris case, this would involve building
- a mind halfway between us and the ocean
- two additional minds: one halfway between us and the new "halfway" mind, one halfway between the ocean and the new "halfway" mind
- four additional minds ... etc.
Implementing this plan in any particular case may involve some "minor engineering difficulties" -- but it's a fascinating approach.
Then there is the sobering possibility that we may have alien human-level intelligences right here on Earth -- cetacea -- yet are unable to communicate with them beyond a trivial level (so far) due to the profound cognitive differences.
Let us search for extraterrestrial intelligences, by all means! But let us not be too anthropocentric in our ideas about what sorts of intelligence to search for. Pattern space is a very big place; minds vaguely similar to humans are most probably a tiny subset of the space of all minds feasible within our physical universe. And the physical universe as we now understand it may be nothing more than a tiny subset of the actual universe.