Thank you for bringing this extremely important point up as it is something that has been mentioned in the past.
The action of the weather often produces aggressive pressures on a roofs surface which would have a destructive effect on loose laid materials and at best, force a route in for driven rain and snow, etc.
Underfelt is a critically important barrier in the make-up of modern roofs and must never be haphazardly breached, at least not without solid, built in safeguards – which a properly fitted lapVent does provide. Therefore, because of the way that a roofs substructure is laid out it would be impossible to fully weatherproof the installation of a lapVent unit without following the suggested fixing instructions and set guidance.
Hope this helps