> Does anyone really need to be warned that they are using the passive voice? Does anyone actually do that by accident?

Yes, all the time. I write a lot and still fall into the trap, and it is hard not to do it sometimes especially if you were only introduced to the concept later in life. I try hard not to use the passive voice, but there are times when I think it is acceptable, and there are occasions where the alternative is more confusing. The quoted speech "Now we are engaged in a great civil war..." sounds perfectly fine to me!

Honestly, I have trouble thinking of an alternative sometimes, even when I can identify a passive voice sentence.