You can say till you're blue in the face there were advantages to leaving it on but the fact remains that in those days people did not switch their answering machine on when at home.
Just because you didn't switch yours off and because you say that "everyone" else in the 80's didn't, means nothing. And certainly doesn't make it "fact".
You can only speak for yourself and the people you know.
I for one would have dialled 999 after getting the call Bamber claims he got, but he didn't. Now because I would have done that, and I believe MOST others would, does that make it fact? No.