Lextek, you raise a good question. Why would people put Blu-tac under their speakers? It is probably because they haven't tried the other way, yet.
Blu-tac and other soft stuff provides a kind of soft "wiggly" interface between items, which at one time was thought to be a good way to do things. Others have used rubber, and sorbothane, etc, to do similar things.
A tweeter vibrates at more than 20kHz, at very low amplitudes, which means very tiny movement of the dome, maybe less than 1/10,000th of an inch in some cases. If your speaker enclosure is "wiggling" at all, then there is doppler induced onto the tweeter(at the very least), and perhaps even cancellation of certain sounds.
"Spikes" became popular years ago to help to stabilize the speakers from this "wiggling" behavior. They also help to couple the speaker to the higher mass of the floor. This can help to absorb the vibrations that are present in the entire speaker cabinet.
Not everyone subscribes to this method. But, I guess that makes for interesting dialogue.
Disclaimer: I work for a company that manufactures audio products, including points and stands.
