I have a Dual Core Intel CPU (C2Duo 6600). I, and some others, did did a few tests.
While ArmA may not in itself be optimized for multi-core CPU's, they do improve my games performance.
Why??
I have an Nvidia 8800 GTX. It, and all graphics cards, still use lots of CPU cycles. While ArmA is hogging one core, my GPU and other stuff, sound etc is using the other one.
Sooooooo, if I only had one core, ALL this stuff would be running on that one core, and my games performance would take a hit accordingly. Don't take this as gospel....your mileage may vary.
This is just how it works for me.