Well to me, the point is: if it can be used spontaneously without extra time by a prepared caster, then the same should be true for a spontaneous caster using the same rod.
Nah, if it could be used spontaneously by a prepared caster, than it should be useable without an increase in time and with unlimited uses per day by a spontaneous caster.
Of course, it would be totally ridiculous, if a prepared caster could use it better than a spontaneous caster.