As I said, the example of protecting (via backfill) a territory which is itself being used to attack makes sense, and I have done so myself on several occasions.
However, I'm still having trouble imagining a scenario where this is a better strategy to help a convoying fleet (or any non-moving unit) hold its position. Let's say your fleet A is convoying, and you order B to move to A and C to support B's move to A. An enemy attacking A with strength two would fail, but strength three would succeed.
Meanwhile, if you simply order B+C to support hold A, it will stop an attack on A of strength three, while using the same number of units. Even if, as Rorschach argues, another enemy unit can break support from B or C, so be it; you're still able to repel an attack of strength two, while technically using one *less* unit than the other tactic.
But whatever...not looking to make a big stink about it or anything... :)