You're the one commenting on the RAM usage.
I am worried about the CPU usage. I would prefer to dump Steam to get an extra frame rate or two.
Steam isn't exactly light, it takes far longer to load than Origin/Live Messnger/MSE ect.
I do hope they learned how to make this title run decently, but I have doubt they did. Hence why less programs = better. I also like my PC clean and only run a few programs in the background. Steam goes off when not in use.
We'll see how it pans out though. The only game I've used on Steam with lots of modding has been Half Life 2. If ArmA 3's overall experience in regards to mod support is easy then there is not much of an issue.