When was he in position to stand in , he's probably never needed to step up. Mainly because HMQ kept such good health, as did Phillip, Charles and Anne.
The issue is potentially now, or in the next few years we know Charles has cancer.
It wouldn't take much for Charles to be too poorly to carry out engagement and for William to get hit with flu or a tummy bug, something realitively minor, at the same time, beyond William its the 3 kids, then technically its Harry and Andrew, Before Edward and Anne.
Its really not beyond possible that both Charles and William could end up ill at the same time.