It's bad because:
1/ It breaks multi-hosting
2/ It's a real pain to setup and even worse to manage if you don't control your reverse
3/ It helps little with spam filtering since all it takes is a single reverse by the spammer to get the expected name
4/ it actually tells you nothing about the nature of the mail, only the way the DNS is setup.
It's pretty standard practice really. Anyone with IP addresses with incorrect/conflicting reverse DNS that is not a customer of that ISP (and therefor not entitled to relay through the server for any reason) probably should not be connecting to it.
And prey, how is an external party supposed to send mail TO them if not by connecting to the machine listed as the domain MX through SMTP ?
If the host in question is another mail server, it should have proper DNS/reverse DNS from day one.
Only in order to follow these rules dating from the day IPv4 was a waste desert and anyone askign for an IP to connect a webcam pointing at a coffee pot would get a class C by default. Nowadays, it's only a legacy rule that serves no useful purpose and that almost every serious player has abandoned. I don't see gmail, hotmail oand yahoo doing it, do you ? that's because they have a real spam filter in place that will actually TRY to avoid false positives by checking the SPF record instead of the PTR.
And I DO have a proper PRT record. It just doesn't equal the mail server's host name.
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