The efficiency/sensitivity of speakers are fairly important when making a decision about what to buy for your existing equipment or new equipment.
As a frame of reference, every 3db drop in this rating doubles the amount of power needed to produce the same volume output as a speaker that is rated 3db higher. In other words, a speaker that is rates at 87db requires twice the power compared to a speaker rated at 90db to achieve the same volume level.
Another thing to remember is that power usually is not what blows speakers (as long as the speaker is rated for that power rating), but it is distortion that kills speaker more often. Distortion increases as you push the receiver closer to the maximum power rating by turning the volume up to closer to maximum on the dial. This is why L2W stated a higher efficiency/sensitivity rating for speakers are desired when you have a lower power receiver.
A good rule of thumb is to never turn the volume over about half what the dial will allow as this is where the distortion will start to get to higher levels that can damage the speaker. If it does not get loud enough, then you either need more efficient speakers with a higher db rating, or a more powerful receiver or amp with the current speakers to protect killing the speakers.
