All designs have different results in mind. I try to make sure all function is on hand and the 5th line is a handy control line for lots of different reasons.
Pictured is why our kites dont have constant tension in the 5th line. It allows the kite to pivot back and forth around the front line attachment point.
It also allows the safety to be spun multiple times and still have a reliable safety release.
When the depowered strap is closed, yes the 5th line is sligthly loose. This lets the kite automatically absorb gusts. Almost like suspension. It lets the arc stretch around 15cm instead of you being effected 15cm. Once tension comes on to the 5th line the kite becomes supported then darts across the window.
There's a history of 5 lines kites ripping
The main reason for this is the single attachment point.
There is a tight area of topsail material (around the size of your hand) right in the corner between the leading edge and the struts. If the 5th is attached by 1 point near the pump leash, when the kite rolls it will runs up and over the canopy close to the tight topsail in the corner and split the kite.

When I was doing kites for Wipika we worked this out after our first proto.
Thats why everyone makes 2 attachments as it is no where near to the nasty tight topsail.
But having the 5th closer to the middle of the kite is better as the kite will pivot from the middle and relaunch better from the U position. Try relaunching a kite with some kind of bridle from the U position and you will understand what I mean by "to pivot or not to pivot"
I could BS on all day but I need to do some other stuff.