If you are more than idly interested in learning about SETI, they have a youtube channel here: http://youtube.com/user/setiinstitute/
They aren't looking for wasteful noise from neo-technological civilizations (but we'll take any unnatural signal we can get!). As you've pointed out, that might occur for a very limited period of time before their physicists move on to better communication technologies. But don't make the assumption that these people are "short sighted," (SETI is 50 years old) or aren't criticizing their own techniques, or actively exploring new ideas. They're looking for broadcast devices, robotic beacons, communications nodes, cross-talk between interstellar routers, gigantic mechanical artifacts, etc. The holy grail would be an alien satellite designed specifically to attract the attention of developing civilizations, like the one in Contact.
There are really good reasons to look in the radio spectrum. The radio range has a noted trough of low noise in 1-10GHz, which is an excellent candidate for communications, if not outright shining a beacon out to the farthest distances. It also penetrates mollecular clouds (and our atmosphere, for that matter) easier than optical and higher frequency light, which scatters easily, and doesn't take nearly as much energy to generate as X-ray and Gamma ray radiation. You will need an EE to further explain.
We may not be prepared to guess at alien biology or sociology, but we do get to project what we know about physics. "They" refers to technologically advanced civilizations.
- Some civillizations may be well beyond anything we can fathom (interdimensional beings, control over gravity, faster-than-light communication). But if they do exist (and that's a big leap), there's very little chance that they are the only other civilizations in the universe. If we make the assumption that "life is out there." We can expect there to be a tremendous variety of life forms and civilizations and some may be within our detection capabilities.
- They all most likely use electromagnetic radiation for communication, since it's the de-facto fastest way to move information through space. Cite wormholes or entanglement if you like, I'm not ignoring the possibility. The algorithms SETI uses pick out general anomalies in known radiation patterns. Even technologies we can't possibly understand have a good chance of emitting some kind of interesting radiation as a side effect.
- They will probably choose communications techniques that are easy to detect against most background sources and will certainly need to be very robust to get through the interstellar medium (charged particles) in the case of an interstellar signal. We might even make assumptions about the kinds of engineering practices another advanced civilization would employ: redundancy, longevity, efficiency. These can help constrain the behaviors we look for.
- If they have at least one outpost beyond their homeworld/star (even a satellite in their own solar system), they will be using directed communication. That means their directed beam is slowly sweeping out a wobbling, circular path across their sky. If we're lucky, and that direction is in line with our star, we might pick up a hint of their artificial signals.
- As the poster above mentions, we are made of some very common materials in the universe and in similar proportions (obviously profoundly more dense on average). That leads to a sound assumption about how life might be elsewhere. At the very least, this is our only /example/ and it's a good practice to focus on planets whose properties are not stupendously hostile to our form of biochemistry.