Yes, a roof vent is an often overlooked detail that can greatly enhance a buildings performance and lifespan. Ventilating a roof is key to removing moisture build up in a roof system. Depending on where you are located an improperly vented roof can cause a lot of problems. From ice dams in cold climates to heat and mold buildup in warmer areas. A licensed contractor can help give an assessment of your current roof and offer prescriptive measures to optimize ventilation.
Every situation in roofing is unique. There is not much need for attic ventilation when there is no attic. The decking materials and the supporting structure make your question one that cannot be answered intelligently without gathering some information. I have been in churches in Philadelphia with stained 3" tongue and groove exposed roofing boards with insulation and plywood mounted above the roof. Even without the insulated deck, a vent in a cathedral ceiling will just add to global warming by allowing an opening for all of your heat to escape due to convection and hot air rising.
Without addressing this ventilation question and getting prior written approval from the manufacturer, there may actually only be a 10 year pro-rated shingle warranty, even if every label on every bundle said 50 year or lifetime.
If on the other hand, there is a cathedral ceiling with a plaster ceiling attached to the underside, ventilation does come into play with the life of your roof and may be required by the manufacturer for long term warranty. In most cases, there is not high humidity in most churches and the decking is so dry from being there so long, that deck movement is not a danger. The three dangers of not having good ventilation are ice dams, mold and deck movement. Sitting in front of a computer and answering the question is an exercise in conjecture, please reach out to people knowledgeable on the subject. Builders, architects and some contractors to start. I have found few experts in the field and I am smart enough to know enough good people to go to to find the answer. Or at least collect intelligent, informed opinions. Construction is so varied that there is no one-size-fits-all answer on ventilation, but educating yourself with more than one opinion on this particular situation is recommended. My normal answer to ventilation issues is balanced and necessary, but you may have the exception to the rule.