It is a challenge to find a definitive and totalizing answer here. For every instance that could demonstrate real corporate value in ethical conduct, another could be found to show it does not mean anything of merit.
Perhaps, one way to examine the issue is to realize the economic costs of being caught engaging in unethical practices. This might be where ethical conduct is valuable to an organization. Penalties are becoming stiffer and more difficult from which to recover in being unethical. At the same time, the use of social media networking and a 24 hour news cycle that continually replays in order to keep presence in one's mind, there might be a significant cost to being seen as "unethical." Certainly, one can make the case that organizations like Arthur Andersen or Enron demonstrate that violating ethics is destructive. This would mean that there is a significant benefit to being ethical in business, as it guarantees long term success and a consistent vision in play. Ethical conduct might be better assessed when seen in this light. There is value in being ethical because the cost of being caught in not being ethical can be disastrous.
If one wanted to be optimistic, corporations that act ethically do gain some value in acting in accordance with stated guidelines and expectations. Businesses that are able to act in accordance with an ethical model that is above reproach is able to use that as part of its corporate image. This can help in generating markets and revenue expansion without ethical clouds looming, This would be an optimistic read as to how ethical conduct can be seen as having value.