The ideal time for a team to employ one cornerback as a shadow of the opposition’s top wide receiver is when it is still trying to decide who the other cornerback ought to be opposite him. If you are not comfortable putting a player out there for every snap, then you probably don’t want him to go up against the best your opponent has to offer.
That is the position the Pittsburgh Steelers find themselves in right now, with Artie Burns still trying to regain his starting job full-time as he continues to be rotated out in favor of Coty Sensabaugh, though he has regained the ‘starting’ distinction and the lion’s share of the snaps.