Marmot, I add line when I am using dry line setup (leech/nymph/dryfly) with 10 foot leader for example, then want to chironomid fish in 18 foot of water. Also in evenings when fishing near the surface, It's easier to use shorter leader. I then add swivel...and more line,,then I can chirony fish in 18'.
I used 3 feet between swivel and chirony as an example. I would be fine with 6 or 7 feet as well.
As for fluoro sinking faster, I agree it does. After lightly stretching better quality mono leader, it straitens and sinks pretty good.
I have and still do use strait leader setups, but often I find the swivel is a good device to make adjustments. but like bbronswyck said, it depends on the conditions and scenarios. everyone has reasons for doing what they do. btw, good insight, it's good to hear others perspectives.