The major benefit to changing a single subnet to a multiple subnet is, in your situation .... Job Security!
You will be working a bit more (Ka-ching!) to make the transition seamless, to troubleshoot it when miscommunication problems occur, and to make changes when your boss requires. It will also be slightly more cryptic unless you fully document your network (as you should do).

Remember some reasons for subnetting: (not in order or completeness)
1) To segregate or group internal hosts based on activity, location, or security. (etc..)
2) To allow for expanding IP device population and subsequent shrinking base of IP addresses.
3) To allow for rapid employee growth.
3) To better manage assets and their logical network communication
4) To help manage bandwidth issues.
5) To help manage road warriors and their needs.

Is any of the 5 reasons above a good indication of why you might want to subnet?

After answering that question, ask yourself, are you experiencing any pains with the way things are, and might become in 3-4 years? Is anyone else complaining of poor performance or lack of being able to do something? Do you see a reason why you should change it?
Why fix what isn't broke, if it isn't broke.

Having a device on a DMZ means there will be hack attempts and the associated pains if that DMZ is compromised. Typically I reserve DMZ's for my troubleshooting notebook and only I plug into it. A compromised DMZ is nobody's picnic, Of course, that's still more job security.

I say if the design is not picture perfect but works well under load, keep it simple.

Just my thoughts.