A small amount of arcing is quite common, but heavy arcing is a bad sign. If they are new motors, they might need the brushes bedding in, if not new, worn brushes will give arcing. Either way, if heavy arcing has taken place, the commutator surface will have suffered damage and at least need cleaning. The arc does damage, the damage creates the condition for more arcing, arcing is a sign that the motor is generating a lot of back EMF. The current flow resulting from that voltage is what gives rise to interference, either by radiating from the motor leads and being picked up by low power wiring (aerial, servo leads etc) or by dumping the unwanted voltage into the power leads. To minimize that sort of thing, motor leads are best twisted so that the unwanted signals cancel out.