Some might say there is more risk involved in disturbing the systems and changing bottles than there is in charging from a remote point - horses for courses
Precisely. The more you pull the bottles in and out, the more wear and tear on the threaded connections(safety), more risks of dropping the bottles from a great height onto the tarmac(safety), the less chance of getting contaminants on the connections(safety), the less chance of personnel getting injured by lifting the cumbersone bottles in confined spaces (safety), much greater turnaround speed (happier pax), etc....