Very good point to bring up on this matter. Speakers are really alot like a fuse. A fuse can take a long lower average power or it can take a high surge amount over a very short period of time...but not both. Same applies to a speaker. I get so sick of marketing hype that manufacturers do and the end user gets burned.
example- Peavey and Yamaha have for a few years been guilty of mis leading the consumer on their speakers. How? ok , here we go..
Yamaha puts a rating on a club 115 speaker as 500 watts program, 1kw peak. Peavey recently had a speaker called a SP218 ( i think) dual 18 sub and it was rated by peavey to be 2k watts program, and 4kw peak. what they fail to tell people is that they can not take this power level for more than about 1/3 to 1/2 a second. and the peak rating is pure crap. It has no musical content and will either cause the speaker to fail by thermal or mechanical means. It might be able to handle the surge a few times at a few millisec each , but not over and over. Speaking of thermal and mechanical failures.. Thermal refers to the voice coil getting so hot that it burns up and can cause the coil former to warp -seizing the cone or the windings will simply fall apart. Mechanical refers to excursion damage like the cone goes out so hard, that the coil leaves the magnetic gap and destroys its self upon reentry- and sometimes blows the amp out too from the dead short. Excursion stress can also cause the glue joints to fail, center of the cone to bust out, cone rips, cone fatigue (it IS just paper - remember?) tinsel leads can be streached and break. If dropped to hard, will cause the magnet to shift which totally screws it all up by locking up the cone and makes recone not possible.
These power ratings are called "program" which the rest of the industry calls the "2 times RMS level". RMS is the continuous power a speaker can take continuously. Anything over that 1/3 to 1/2 sec time frame is considered continuous to a speaker. Program is really a marketing scheme to up the power rating to the consumer. You should amp your rig to be 2x the rms rating of the speakers, its for that punch and dynamics. Great for classic rock, but what if your doing country, rap or heavy rock that is extremely strong in that low energy content? you can see that a kick drum signal only lasts a fraction of a sec. but you add in a sustained bass note that is also very strong and poof - blown speaker. THIS is where a loud speaker processor like the DBX Driverack comes in to play, it has a feature that you can limit your rating to the continuous, but pass the 3db (or more) punch when its needed.