Rejecting on broken or non-existing DNS will probably reject mail from more than 15% of all mail servers on the Internet - guaranteeing a false positive rate not even matched by the combined 6 DNSBL's I use - cumulative and with hard 5xx rejects. AT&T on the other hand, will use DNSBL's when the first snowball emerges from hell unscathed.
In the good old days, when network engineers used VT100 terminals and 300 baud (not bps) acoustic modems, ftp.uu.net enforced the requirement for "valid" reverse and forward DNS entries for anonymous FTP access. It was sometimes the only way I could convince customers to type the line in both DNS files. If you don't have valid Address<>Name mappings, you won't be able to download files from ftp.uu.net. Doesn't anyone else find it funny when people scream that ISPs should block ports and shoot people with misconfigured systems; yet when an ISP actually does enforce even a modest requirement; people start screaming how unfair or stupid that ISP is for doing that.