Injecting false positives generally can impair quality and whether or not quality will be impaired or improved with false positives is really context dependent. Indeed, low false positive rates are often used as a measure of quality, so in generally you don't want to increase them carelessly.
In the case of things like phishing training, I imagine (but I could be wrong) that the injection of false positives just causes the people who recognize phishing emails to ignore them instead of reporting them: there is too much noise and too little signal. The people who don't recognize them will continue to fall victim. In that case, inuring the knowledgeable seems detrimental since you lose the likelihood of receiving a report.
I follow inbox zero practices and routinely delete all my email. Since forwarding a phishing email to security is a lot more complicated then hitting the delete key (like I probably just did for another email) I'm personally most likely to delete phishing emails unless I am getting them very rarely or it seems especially pernicious. Indeed, most of the phishing emails I receive lack a certain phishy feeling (like lacking a DKIM signature or other weird mail header shenanigans). I generally just assume they are these sorts of false positives.