SETI Rationale Difference & Philosophy
We can suggest that the optical signals are strong since so far, a systematic optical search has yet to be conducted. There might be strong ETI signals present in the optical spectrum, for it could be argued that we should have discovered their microwave signals by now if ETIs were using "magic" (powerful) microwave technology.
Also, optical communications are likely to become very mature and the main means of communications on this planet within the next few decades, e.g., fiber-optics to the home, optical computers, and GEO to GEO/LEO orbital links.
In the coming decades, despite concerns of the SETI community about radio frequency interference problems, the radio brightness of the Earth may drop drastically as the more efficient beamed optical technology becomes dominant. It will also become our main communications technology across the solar-system and deep-space. This will be the technology used in interstellar space, as we send out non-relativistic probes to investigate and relay back pictures and data about our nearest star systems. Early next century, men will walk on Mars. That second "small step for a man . . ." will be seen by mankind via HDTV signals beamed to Earth. These signals will probably traverse most of the distance between Mars and Earth on an optical carrier, and may be relayed around the globe via direct laser links between geosynchronous communications satellites. Many viewers will receive these pictures via fiber-optic Cable TV.
The world of the future will be dominated by photonics (optoelectronics). It is very unlikely that microwave technology will remain the dominant form of beamed communications. It is more likely that for the greater span of the lifetime of a technical civilization, that photonics will have the greatest impact on society. For this reason, if our civilization is typical of emerging technical societies, it would appear most natural for ETIs to communicate via optical photons.