I think Geoff's hit this nail right on the head!
The little I've heard of airchecks of “Fresh” and “Now” seemed to “burn out” these “insert-shouts” (hard or “whisper”) literally hearing them half-a-dozen times
(unless of course the “consultants” think that this “is subliminally building the brand conception awareness in the audience“!).
In the past, many US stations would consider replacing their entire “traditional” (and expensive) packages annually (and even down to 9-month intervals) because they could “burn out” and become stale to their listeners in that time.
(Any other comments, I leave to others…(e.g. Radio Two jingle enthusiasts!)