Try this:
I'll skip the simple argument of "recreation". I'll also skip the analogies such as drag racers, "why does anyone need that much horsepower".. etc.
Your question is a valid one and deserves a valid lucid answer.
First:
At the core of this debate is the Second Amendment. Many that have posted, including me, believe that the while the Second Amendment grants certain rights, that responsibilities are imparted with those rights.
"A well-regulated Militia being necessary to the security of a free State,
the right of the people to keep and bear Arms shall not be infringed." - Second Amendment
You will note that it says a "well-regulated militia". This is one of the responisbilities that I believe are core to the Second Amendment. While firearms ownership should be protected at all costs simply because it is a God given right to protect oneself, the responsibility to have the skills necessary to employ your weapons in the defense of your self, your family, your country and your freedom is paramount. This clause exists to remind people that freedoms must be protected at all costs. Threats may come from outside the government or inside. It is a catch-all phrase insuring that this nation remain "of the people, for the people and by the people". The Second Amendment protects the right to own assault weapons, and applies the responsibility of using them to combat threats to our freedoms both foreign and domestic.
Second:
These weapons are used in nationally recognized sporting events. International Practical Shooters Coalition (IPSC), International Defensive Pistol Association (IDPA) and numerous local and national organizations promote the sporting aspect of shooting. Not only that, but they also practice skills at arms and tactics which help satisfy the Second Amendment responsibility I mentioned above. When competing in these events, generally they are scored based on time to complete a course or a set of actions. If you ever watch one of these competitions, you will immediatly recognize the immense benefit of having firearms that can fire rapidly, accurately and with as few magazine changes to waste time as possible. Here is a short video clip of a young man competing to illustrate my point.
http://www.krtraining.com/IPSC/Matches/2002/TxStateLimited/stages/stage5/NATHANSt5.AVI
Third:
And quite possibly most importantly, self defense. Military small arms (pistols, shotguns, assault rifles) are designed so that they may be employed by a wide variety of troops. They are specifically made to fit the average sized individual, or with minimal effort, be modified for use by larger or smaller than average individuals. They are made to be maintained relatively easily. They are made to be durable, reliable, cost effective to produce and effective. When not otherwise employed in sporting or hunting roles, military small arms are primarily used to protect something by applying deadly force to neutralize the threat to life, liberty and the LAWFUL pursuit of happiness.
Because military small arms are designed with these considerations, they are extremely effective in a self defense role. Because of their simple operating procedures, I can, with minimal instruction, feel confident that my wife (or other family member) can employ deadly force in self defense. I can be fairly assured that the application of deadly force with military small arms is going to be effective at neutralizing threats to my family. I cannot be so assured with other hunting or sporting specific firearms.
Conclusion:
The media is correct in saying that assault weapons are "killing machines", just as I am correct in using the same term when refering to automobiles. To say that they serve no purpose in civilian hands is the cry of the uninformed. As law-abiding Second Ammendment proponants, it is our job to protect the rights, priviledges and fulfil the responsibilities I've outlined above. I really hope that at some point you might join that fight considering that it is the application of deadly force by military weaponry that guarantees your right to question thier legitimacy.
I hope this helps.
WolverineAtWork