depends on what you mean by 'top-cover'? You'd have to ask the Sea King boys if they valued our presence on 'edge of range' trips. I know we got thanks on more that a few occasions when we could assist with updating vessel positions and even establishing comms in fluent franglais or spanglish
Also ask the all-female round the world yacht crew who had useful supplies dropped to them from an MR2 handily on detachment nearby. In most cases, I'd say top-cover is maybe not 'essential' but certainly 'useful'