Sticking around waiting for a client to rediscover its soul is a risky move. Depending on the nature of the client relationship, though, it could also be an opportunity. As a trusted partner, an agency can help the process redefining the client's corporate meaning and mission.
Case in point: GS&P is quitting HP.
I personally disliked their TouchPad campaign. B-level celebrity rich, light on actual meaning. Outside of this campaign, their HP work was pretty darn cool. Bold move.