Revealing manuscript referees' identities after review: Why is this a bad idea?
Note that I can see pros and cons of optionally permitting reviewers to disclose their identity, or even making peer review completely transparent. But the following focuses on the problems of disclosing reviewer identity after a decision has been made about the manuscript.

It creates a conflict of interest for the reviewer.
There is an incentive to provide a more positive review. For example, there is the possibility that the author may be more positively disposed to the reviewer in the future, if the reviewer provides a positive review. Also, the reviewer may not want to damage an existing relationship.

There is little difference between perceived conflict of interest and an actual conflict of interest. Thus, even if the reviewers are not influenced by this disclosure, others evaluating the integrity of the publication may perceive a problem. 

It can create more work for the reviewer. If a reviewer knows their identity will be disclosed, they may have to spend more time being tactful. It's hard enough getting reviewers, if the workload of every review increases because the reviewer has to spend more time being tactful or worrying about the consequences of their review on their reputation, then this may make it even harder to find reviewers.