Importance of PTR records on nameserver IP's?

I have not yet experienced any DNS failures due to a missing PTR record, but am wondering:

Is there any added value in having a valid PTR record on a nameserver IP-address?

There shouldn't be any real functional issues due to a missing PTR record for a DNS server.

It may come in handy if you're dubugging network problems, but even then, I would think it would have limited usefulness.

It's extremely important if you are running a mail server. Otherwise, it's not important (IMHO).

Why? Many spam filters consider a missing PTR record as highly indicative that you are a spammer.