If these are brushed universal motors then the frequent starting and stopping will cause excess wear on the brushes. If indeed there is fine dust working its way into the motor, then the dust will coat the brushes and armature causing more electrical resistance, causing wear. And, if the fine dust is collecting on the internal motor fan blades, then the cooling airflow will be reduced (10% change in the blade geometry can result in a 90% reduction in airflow). In addition, starting and stopping frequently can prevent the cooling system from effectively removing heat from the motor, causing overheating. Add all these things together and you have a recipe for disaster.