Few words about the AC3D, Blender, scripts and NGONs
AC3D 6.0.30
A native .ac format and OE is 100% compatible with it. No import/export actions needed than OE's own Convert/ Export.
The NGON's and polylines are supported.
Blender 2.42A
Different format require a real import/ export action to /from ac format.
The NGON's and polylines are NOT supported.
Scripts (made with python)
I made AC3D export script for Blender, which can manage the hierarchy tree and properties very well. The process is much easier than in AC3D.
So far I didn't have a time to take a look to the import script. I only fix one bug from the original code and script works somehow with certain objects. Naturally 'NGON'ed' objects can cause errors or strange looking results like over lapping edges etc.
NGON (polygon with 5 or more edges)
This AC3D .ac format feature is beneficial in custom object flooded maps and especial the scenes like Leningrad in Karelia map.
The NGON's are reducing the total numbers of the surfaces and also the objects memory foot prints size. Both have important role in optimizing.
If you have just some custom objects in your map, you won't see any differences between the NGONed and not NGONed objects.
I do modeling with Blender but I often verify the export result in AC3D (trail is over).