The big advantage should be reliability, reliability and more reliability. And smaller floorspace/power/cooling needs compared to a large number of racks of lesser machines, which over time can (I am told) be a significant benefit.
For development, and maybe for rapidly adding new 'servers' if you havs a 'one server per customer' environent, creating extra virtual linux machines would also be very handy, I suspect.
You'll quite probably be the first person in the universe to port AOLserver to Linux/S390, as well as trying out OpenACS there. Sounds like a lot of fun
Make sure you post the results of your experiment here!
The only possible 'problem' I can think of would be that you're running a newer Linux port, will need to recompile some software on that new platform, and there are less people with experience of Linux/S390 than Linux/x86 you will be able to rely on for help.