Sigh...
Here are the main reasons you actually WANT back EMF in a motor. First point to remember is that there is no such thing as a man made perpetual motion device. Remember this as i explain why you NEED back EMF.
By now everyone reading this thread will know that when a magnetic field passes through a conductor, an electrical current is produced. Conversely, if you run a current through a conductor, it can create an opposing force to a nearby magnetic field. Faraday loved this notion, because the functions are totally reversible, which strangely enough fitted in with his religious beliefs.
So, imagine we have a motor. We apply a current to its wound fields, they create an opposing force to the fixed fields. The details of construction are irrelevant. Say this motor is 12V, like the Steorn demo motors. The fields have quite a low resistance. So when you apply the voltage to the fields, there is a high current draw. Lets say the fields have 1 Ohm resistance. Voltage /current x resistance. You have 12 amps of current.
But hang on, lets go back. Remember that when a magnetic field passes through a conductor, an electrical current is produced. It doesn't matter that you have just applied current to create a motor. Do you not still have a conductor passing through a magnetic field? Yes, you do. Even as the device is functioning as a motor, it is generating a current. This is back electro motive force. So what of it? Well, naturally the back EMF is less than the applied voltage. But lets have a look at what happens (using ball park figures for demonstration purposes).
Situation 1) Voltage is applied to motor. At rest it is infinitely slow. V=12,R=1, A=12
Situation 2) Motor is now spinning at say 500 rpm. It is producing some back EMF. Say 1V. So now total voltage applied is 11V (12V-1V). So now V=11, R=1, A=11.
Situation 3) Motor is now spinning at 1000 rpm. Back EMF is now 2V. So now V=10,R=1,A=10
Situation 4) Obviously this is an infinitely variable scale. Lets just skip to 10,000 RPM. Back EMF is now say 11.5V (in real life nothing is linear, there is always some sort of curve) So now V=0.5,R=1,A=0.5
edit: i should point out that situations 1 to 4 normally happen in a split second. Very desirable in a lot of cases. For instance, at rest a car starter can draw up to say 500 AMPS. When it is spinning, with no load, maybe 20 AMPS. With a load, say 150 AMPS.
This is both what an electric motor does, and what we desire. Because there is no such thing as a man made perpetual motion machine. Why do we desire this? Because the current draw is directly proportion to the torque produced. Therefore, Our little electric motor has full torque at rest, and least torque at high RPM. This is exactly what is needed in most cases. For example, an electric car, an electric drill. A starter motor for a petrol engine.
A motor is a motor is a motor. They are there to fulfill real mechanical needs. Most of our needs for electrical motors demand torque. Not all mind you. ceiling fans have very little torque. They use induction motors, utilising the single phase a/c mains current instead of a commutator or speed controller. Because of this, they have very little torque, since there is only a narrow range of speeds the motor is designed to work in.
I would bet you a dollar that when they applied a load to that 'regular' motor in the video, they probably used say two fingers on the shaft. And i bet that for the 'perpetual motion machine' (lets call a spade a spade) they could have put a fingertip on top of the shaft for the same speed reduction. That's how little torque that thing probably has. Therefore it takes very little to keep it going. But what's the point of that? Take away the power and it will take longer to stop than a regular motor. Maybe even magnatudes longer. Have they invented the ultimate 'it takes a long time to stop' machine? No, my 200kg flywheel would trump it...