It's already been pointed out that lame delegations are more likely problems for many. But the "we'll just pre-fill in-addr to avoid problems" isn't going to work for ip6.arpa. If anyone has enough hardware to serve the zone for a /48 (64k * 4bil * 4bil * bytes-in-record), I'd love to see it. :)
If PTR exists in zone file, serve it. Else, synthesize generic reverse. Jobsagoodun.
We need to get web and app folks to stop counting on ip6.arpa/in-addr.arpa as a validation of trustworthiness. PTR make some sense for validating servers, MTAs, etc. and it's handy for traceroute but it was never a great tool and it's getting less useful with time.
I've always seen it as a reasonable indication of a) minimum level of clue and b) giving a damn. If you can't be bothered or don't know how to provide even basic generic rDNS for your network, there's a reasonable chance you're lacking in other areas of network / user management. (Not "you" personally, of course). Regards, Tim.