The question should not so much be whether SETI is a good idea but whether most people even care and considering the possible global consequences it may not even be advisable. Apart from the vast scientific community, individuals with an interest in science, and those people who question our place in the universe most people probably don't care whether the SETI program exists. Most people in the third world do not know of the existence of SETI and the program does have the potential to affect them as well if an advanced alien race is discovered.
Consider the people factor, do most even care. The SETI program really hasn't made a compelling argument justifying it's existence and produces even fewer scientific or technological results that would appeal to the public in general. The program lost it's greatest and most vocal proponent, Carl Sagan, several years ago and has failed to produce a compelling spokesman since then. To most people Carl Sagan was the face of SETI for many years and he possessed a charm that was capable of sparking a scientific interest just about any person. Since his passing, however, there has been a marked decline in advancing the SETI program.
In the present day most people believe that the money used to finance SETI could be spent in better ways that produce more substantial results that benefit our modern day lifestyle. Unfortunately, SETI has a limited potential to advance any technology except for radio astronomy and possibly computer science, therefore, a lot of people would believe that it is a waste of resources. Advancing knowledge for knowledges sake is a noble pursuit but does little to serve any practical purpose to the average person.
Another reason that SETI may not be a good idea is not as down to Earth as the previous argument but is of extra-terrestrial origin. Would it necessarily be a good thing if an advanced alien civilization became aware of our existence? Unlike the worlds of Star Trek it is doubtful that an advanced civilization would simply come here to say hello and share their technology. It is also unlikely that such a civilization would come here to enslave us considering that we would have little to offer them.
More likely it would be like a seven year old discovering an ant colony. At first the seven year old might simply observe the entrance in the ground. After getting bored the seven year old may just go off and do something else or may decide to see what happens by antagonizing the colony. What happens if the anthill is blocked up? What happens when water is poured on it? Shoving a stick into the opening?Introducing another bug into the colony? Taking a magnifying glass the ants coming and going?
At first glance ants seem to have some rudimentary intelligence considering how they work together to build their colony and for the most part we ignore ants until they become pests (unless were a seven year old). If an alien civilization became aware of the Earth it would most likely think we were no better than ants and would probably treat us as such. They would probably simply ignore us or might try to antagonize us like the seven year does to the ants. In the vastness of the universe our planet is very small and any interaction with a sufficiently advanced civilization will undoubtedly affect humanity on a global scale.
Despite this writers speculation the question of whether the SETI program is a good idea is still at the forefront of the debate. On a personal level this writer has in the past supported the SETI program with a subscription to the Planetary Society newsletter. I no longer subscribe to the newsletter but I still believe in the goals of SETI because I would like to know if there are other civilizations out there. I do not, however, believe that any government should spend taxpayer money on supporting such a program, unless the program can be shown to produce scientific and technological advances that are of benefit to society. Personally, I believe that listening for extra-terrestrial life is fine but it is far to risky to initiate a contact with such lifeforms. We have been leaking radio and television signals for nearly a hundred years and we leave all kinds of junk in space, eventually somebody will find us and we'd better pray they just want to observe us and go home.