The alleged motor neuron system was accidently discovered by scientists in the 1990's who were studying the cortical activity of monkeys whilst performing certain motor tasks, one included clutching for food. One day a researcher went to grip his own food and noticed the electrode output from the observing monkey spiked. A link was hereby put forward between watching an action and the same neural system activating as if you were performing the action.
Perhaps it was at this point when psychologists became excited and begun to spin webs using mirror neurons as the fabric. Mirror neurons became quickly tied into empathy, motor action and various forms of cognition.
I find that when something new comes along everyone gets perhaps a bit too excited, jumping to conclusions and correlating A to 49. Mirror neurons also were the subject of study when it came to Autism - do those with autism have a different mirror system? It was also linked to Psychopathy as a possible explanation to the standard deficit in empathy.
I guess the title of this post is a bit misleading because mirror neurons probably do exist, either as distinct neurons firing when observing action separate from other neurons or as a behaviour of all neurons in a system regulated from a different cortical system (Now I'm the one speculating).
My original investigation into motor theories lead me to a perhaps greater topic - the speculation behind science. Perhaps until a unified theory of the mind is theorised psychology will always be flawed. Judging from the journey Physics took to become as close as they are to being united under string theory and the sociological influences that pushed them there, it might be a while before psychology catches up.