Not true: lots of ISPs don't bother to create in-addrs for even private highspeed connections (e.g., cable modems), and you'd be surprised how many medium -> large sites don't have in-addrs for their outbound email relays :)
And they would be denied until they do. There is no way to fix this problem without some pain to somebody out there. What we need to find is the low hanging fruit and reverse lookup is one of them..
When I set up my mail server it took less than a day to discover the importance of IN-ARPA-ADDR registration. Most ISPs don't do a reverse lookup but a significant number of corporate mail systems do, which I found out after about 25% of the outbound traffic got hung up in my server.