My short answer is : Brushing your tooth without toothpaste will not cause a greater amount of micro-scratches than if you were to use toothpaste.
To begin, tooth paste, is a product most often made of abrasive substances that increases the scrubbing power of the toothbrush bristles. Some kinds of toothpaste can also accomplish other things, such as reducing dental sensitivity (potassium nitrate, Sensodyne®) or make teeth more resistant to caries and acidity (Fluoride).
In other words it is the physical movement of the bristles of the brush that removes plaque that accumulates on and between the teeth and gums.
Therefore, it is not the tooth paste that will make the greatest impact on your teeth and gums but the way you move your toothbrush on the surfaces of your teeth and gums.
If you brush too aggressively, you will at first cause the gums to recede and then will use prematurely the enamel on your teeth.
Here is a list of things to do to brush properly:
- Brush your teeth by using the modified Bass (roll-brush) method. Keep your toothbrush at a 45º angle and make small circular movements, on a set of 2-3 teeth at a time. Avoid broad, horizontal, back-and-fourth movements, since they will irritate the gums. Once a set of teeth has been brushed, make one sweep from the gum to the teeth, to remove any debris stuck in between.
- Don't put too much pressure, and use a soft or super-soft bristle toothbrush. If done correctly, the plaque and bio-film will be removed by the movement of the toothbrush bristles. Anything that is harder than what a soft bristle can remove is probably tarter (calcified plaque) which needs to get removed by a dentist or hygienist.
- If you need to remove food stuck between teeth, use floss.
A more in-depth look into mechanical plaque control could be done, but it would be somewhat outside the scope of the original question.
One of my references: