Ideally, resistance should be higher than the bulb. A 5W bulb would be about 30 ohms. The resistor doesn't isolate the D+ circuit, but it does limit current. A 30 ohm resistor will limit current draw through the D+ circuit to less than half an amp under the worst conditions.

Damage may not occur for a very long time. But if there's an unusual condition in the system...a weak primary diode, a short, or a completely flat battery...it can be quick.