Assume I own the domain name, example.org. I am creating a setup where I receive email sent to firstname.lastname@example.org using Google Apps. The MX records for example.org already point to Google's servers and it works great. Now, I have an application server where I would like to send out automated emails, such as notifications about user activity, etc. I may also use this server to send bulk emails such as newsletters (legitimately, of course). Here, I have the option to use a subdomain for all of my automated outgoing email. Perhaps I can send these emails from email@example.com and set the Reply-To header to firstname.lastname@example.org, which would then be directed to my Google account for a real person to answer.
My question is: Is there any added benefit of sending automated email from an email address with a subdomain?
Note: I have already taken measures to use SPF and DKIM to authenticate my email from the application server. I have heard that some receiving mail servers also do a reverse MX check to determine whether your email is spam. If this is true(?), I'm assuming a subdomain would be useful because I can then make a separate MX record for mail.example.org to point to my application server, while keeping the other MX's for example.org on Google.
Aside from that, is there any other reason I would use a subdomain? I notice this is commonly used in email from Groupon, Southwest Airlines, Office depot, and countless other newsletter services (sometimes they use entirely different domains), and I haven't been able to figure out why.