real life reasons not to use reject_unknown_client_hostname

The documentation[1] and several e-mails here mention that
reject_unknown_client_hostname can reject legitimate e-mails.

What exactly are these scenarios? When do they occur in real life? Are
there really legitimate mail servers that don't have a reverse DNS
record that resolves to their IP?

I would like to know so that I can decide whether I should care and
whether I can use this option for my setup. I would only use this option
for port 25 (not submission) and make sure that sasl_authenticated
clients are exempt from it.