While asbestos poses an environmental concern in any form, it is most hazardous when it is disturbed, causing the particles containing the small, barbed fibers to be released into the air. Should you choose to have the material removed, you must hire a licensed asbestos abatement service.
I would not discourage the collection of rainwater on any roof surface as long as the water is utilized for non-potable use such as landscaping and car washing. The danger of cancer causing particles being inhaled in minimal in the presence of water. Not trying to downplay the hazards of exposure to asbestos, keep in mind that asbestos does occur naturally in the environment.