Military teaching, at least when I was directly involved in it, was that extending downwind is a big "no-no", for reasons already stated. Once someone does it in a busy circuit, the pattern tends to get longer and longer.
I was once instructing at a University Air Squadron Summer camp in Scotland. We were required to fly circuits at a civvy airport. We were flying two of our circuits inside the huge circuits being flown by some other pilots in light fixed wing.