well, from what i hear:
they originally began with one key for everything. fairly simple. one key with a few cuts for security.
well, people started complaining that when they left their car with a valet, there were items missing from their trunk. also, theives had managed to make a mold of the outside lock and presto, they have an ignition key as well.
so they went to two keys, 5 unique cuts on each key. this was good because now the ignition key was unique and making a mold of the door lock would not produce an ignition key duplicate. also, one was able to give the valet the ignition key and not need to worry about what was in the trunk.
then, the finicky public stepped in and started complaining about having to worry about two keys for every car, and locking keys in the car, so they went back to one key. but this time, the key had 10 cuts. five cuts were for the door lock and the other five were for the ignition. by having a ten cut key, a theif could not make a mold of the doorlock and use it in the ignition. the reason being: the mold of the door lock would only yield the 5 cuts needed for the doorlock. the cuts that were needed for the ignition were entirely different.
usually, when there is only one key required, the manufacturer will also include a valet key, which looks the same as the normal key, but it only has the five cuts necessary for ignition. so that key would not work the locks or the trunk.
i would have assumed that the sky would go with the one key, however, i wanted to get your opinions on that.