Here is some information about the external wind baffle.
Generally, leaves and debris that can fall in behind the external baffle are not a concern because of the low pressure (wind effect; Bernoulli Effect) capability of the ridge vent making it essentially self-cleaning. In the AirVent video (on our YouTube channel) during the wind driven rain tests you can see there is a significant amount of low pressure created which can draw water up and out of the vent, so leaves and debris are not an issue. Another benefit of a product like ShingleVent II is that it does not have any external ribs perpendicular to the external baffle. This allows the leaves or debris to blow down the vent parallel to the ridge and eventually be blown out the end of the vent run.
In cases where there is little or no wind (middle of a dense forest) pine needles and debris that fall and stick behind the baffle will not be pulled out. In these cases shingle manufacturers require that the roof be cleaned off because the pine needles and debris left on the roof can degrade the shingles prematurely. If the roof is being maintained for the shingles, the ridge vent should be cleared at the same time. In situations where there is little to no wind, another option is a power attic ventilator.