That's a good point, if we expect to eavesdrop on functioning communications systems employed by ETI. OTOH, If ETI is interested in sending out a hailing signal, they will be guided by physical, not technological, constraints. The galaxy is very quiet, and extremely transparent, at certain microwave frequencies, and we would expect any advanced cultures interested in astronomy to have developed technologies to monitor those frequencies, or at least, harmonics of them. This is a logical place to operate whatever the tech capabilities of both sides are. Even if ETI routinely communicates with subspace radio or telepathy, they'll still listen on the 21cm line to map the galaxy like we do, so 42 or 10.5cm are logical places to listen for their hailing signals. There are a variety of other narrow lines in the high SNR microwave spectrum with similar properties: quiet and clear bands with non-SETI applications probably familiar to all species able to build interstellar communications equipment and with an interest in the interstellar medium. So far, I believe traditional SETI researchers are on the right track here.

What is highly unlikely is that they will be aiming a signal directly at us, which is why I've always felt optical SETI, or any monitoring of nearby sunlike stars for tightly focused radio beams, is a waste of time.

It may very well be true that the galaxy is saturated with communications we don't know about, or which we cannot identify as artifacts, but if that's the case, there's not much we can do about it.
It's not dumb to look for your car keys in the pavement around the streetlight if it's too dark to see in the tall grass around it. You have to start somewhere.

There's a post by ljk (5/8/06) on this forum covering some of these issues in some detail.