Couldn't this result in darksend transactions being exposed? If someone denominates, darksends, and then denominates again at a later time, the transaction for the second denominate will likely include both the change address for the darksend, and addresses used in the first denominate. This would link the change address to the rest of the wallet, and since there is likely to be only one corresponding output in the darksend, the darksent portion could be identified.

Even if that couldn't be done, denominating the whole wallet together allows anyone who knows one receiving address to identify most if not all of the other receiving addresses. It seems to me treating each receiving address as separate instead of operating on the whole wallet would be a safer, although more annoying, choice.