Mike Adams wrote:
More likely is what others have said - by staying
on the centerline, they have more flexibility to
sequence traffic on the left downwind, and others
can spot you easier if you're in a predictable
location.

Others spotting you easier where most would look if they hear someone is making
a "straight-in" approach is the best reason, IMO.

Something similar happened to me today at an uncontrolled airport that brought
this discussion to mind. As I was about to enter the 45 for a left downwind, I
heard another aircraft announcing that they were on the left downwind. I
scanned the entire area but did not have them in sight. I announced when I was
on the 45, still looking. As I entered the downwind, I announced and asked the
other aircraft for an update on their position. He said he was on an "extended
downwind, about to turn base." I looked everywhere, but expected to see him
more or less way in front of me and/or to the left as he made his left base
turn. Instead, I finally saw him coming across from my right (I was still on
downwind) making what I would describe as a *diagonal* combination base/final
straight from some point WAY out to my right!

Of course, he was considerably further than 30° off of the centerline, but more
important was that his description wasn't accurate for where he was or what he
was doing. IMO, he was on a *WIDE* downwind, which he didn't say, and he didn't
"turn base," he just flew a diagonal line to the runway! I interpret "extended
downwind" to mean traveling further downwind before turning base, not flying
the downwind 1.5 miles away from the runway.

If I were an ATC today, I sure would have said something, too!