Speculation is basically a guess, and guesses are nothing more than gambling. A scientist's time is much too valuable to begin extensive research on a highly improbable outcome. As the initial impetus for an idea, there is nothing wrong with speculation--scientists do it all the time. However, before they promote that speculation to the level of a hypothesis, they make an evaluation to determine whether the concept has a sound scientific basis--if it does not, then it is discarded. A scientist would quickly lose all credibility if he promoted ideas that were based solely on speculation that ignored scientific principles.

Your question about the String theories makes a good point, but as far as I know, the theories do not violate known principles of physics--rather they provide logical theories for observed behavior that help explain some of the current enigmas about nuclear physics.

My problem with the speculation about sensitives is that there is essentially no scientific foundation for such a belief, yet there are people who claim with absolute certainty that their symptoms are related to earthquakes. The fact is that there are known causes for those human physical responses, yet the sensitives prefer to attribute them to the unknown--that amounts to basing their theories on faith, which, although sufficient reason to support a religion, cannot form the basis for a scientific investigation.