Nobody who actually cares about reliably receiving mail should depend on implicit MX. Of all the things mail administrators have to do these days, like reverse lookup, SPF, and DKIM, is it reasonable to care about anyone who can't be bothered with MX records?
I'd gladly reject such domains if doing so improved overall mail delivery rate to popular domains. And unlike 99% of situations where an email fails to show up, the sending web site can instantly report the reason for failure.
I manage e-mail systems with thousands of users and a fair number of domains as well. In most cases, I also manage the authoritative DNS servers for these domains so I make sure that all the appropriate records (MX, TXTs for SPF, DKIM, etc.) are set up properly.
You're right, though. If example.com hasn't bothered setting up an MX record for example.com -- even if mail is hosted on the same machine as identified in the A RR -- they probably aren't worth worrying about. It seems reasonable to conclude that they really aren't too concerned about being able to receive mail.
