Speakers are damaged by exceeding their thermal or mechanical limits.
A common way of damaging speakers is to send them a clipped signal from an amplifier. Clipped signals resemble square waves, and square waves have higher than normal levels of high frequency energy. It is that energy which causes the tweeter's voice coils to heat up and possibly to damage them.
Which is why people will recommend you have more power on tap then is needed, so that clipping is avoided.
A hotly debated topic is how much power do you actually need? It depends on your speakers, your speaker placement, your room and your personal taste.
I did a recent test, that some people seem to disagree with, using a Kill A Watt meter. It measured a peak power draw of less than 300 watts total when I was driving seven speakers and playing the opening scene of the movie Iron Man. Personally, I think that says a lot about how much power is needed in real world listening. I think it says you don't need as much as people think. But that's just my opinion.
