The electricity flowing through speaker wires is not constant, but rather always changing. A light bulb is not designed for this. By an electrical phenomenon called impedance, which is similar to resistance, a light bulb changes the characteristics of the electricity. The end result is that the bulb distorts the sound coming from the speakers. This is not good, since the sound will be raspy and rough, instead of clean.
The more electricity that is run to a light bulb, the brighter it gets. In the process, its resistance increases as well. On the surface, this sounds like a good idea for speaker protection, but in fact it is not. First, as the bulb gets hotter, the voltage level is reduced at the output, causing muffled sounds through the speakers, particularly in the high frequency tweeters. Second, calculating the correct size bulb to use is next to impossible, since bulb design engineers never designed the bulb to be a fuse. The temperature versus resistance curve can vary widely from bulb to bulb, even though similar bulbs are labeled with the same wattage.
A light bulb, if being used like a fuse, will blow when the wattage rating is exceeded. However, what destroys speakers is a combination of two things: wattage and waveform. Exceeding the wattage will cause the speaker to, in layman's terms, "blow." Exceeding the waveform specifications will also cause the speaker to blow. A light bulb introduces harmonic distortion into the speaker.
Think of it this way. Two ocean waves are heading towards each other. When they meet, the collision forms a wave twice as tall as either of the two waves. The light bulb introduces one wave, called the harmonic distortion. The peak wave formed is called the total harmonic distortion, or THD. The light bulb, instead of being the speaker's protector, in reality becomes the speaker's destroyer because of the induced THD.