When you log in to a site, all the major browsers automatically ask you if you want to save your password. But if you display the password in plain text then the browser doesn't detect it as a password... and it doesn't offer to save it for you.
So what is that likely to result in? • Users getting annoyed at your site, and thus lost business • Users writing down their password somewhere else, resulting in a loss of security
These outcomes are exactly the opposite of what Jakob Nielsen is trying to achieve!