I didn't read every comment, but the general theme seems to be that it's not absolutely required to have a reverse DNS entry. While this is true per the RFC - it's incredibly bad in practice. Google MX check, run the checker and if you don't have reverse DNS it'll point it out. Also people that say not one mail has bounced because of this must simply be wrong. Many blacklists will auto-add you if they notice you don't have reverse dns, then many companies will pick that up. Last time i had to move my companies mail server, the reverse was inadvertently not setup properly - not only did this cause problems fairly quickly it was slow to fix because while you'll be added to blacklists instantly, getting back off them is a manual process - you have to find every one you are on and then the companies that have picked up this info then have to get the new info - and some don't do this in a timely manner.
Running a real mail server for a real company without a correct functioning PTR record would be something that should get you fired.
The reasoning is simple, anyone running a real mail server will easily be able to set it up, if you don't have the PTR it likely means you are a spammer or you are running a server at home. Not that there's anything wrong with running your own SMTP server, but that's basically how botnets send spam...so there's a heavy correlation to that and spam.