Ok I gotta throw in my 2 cents here as a C "ish" rider. That LOVES having a 5th line
I want 5 lines......here's why,
1) As mentioned it makes the kite go a bit better. My biggest observation once I started riding kites with loaded 5th's is that they went upwind a heck of a lot better. When I'm at a crowded spot, I wanna launch, and get upwind of all the newbies and lawn mowers so I can start to jump learn new shit, and ultimately crash a lot.
2) I crash a lot learning new stuff....UNHOOKED.... I usually do this in shallow(ish) flat water, when leashed suicide, if I screw a trick badly and lose the bar, kite tomahawks with a BOOM!....kites only have so many of these before shit starts ripping and breaking.....sometime only 1 really good one. If I know that on something like a handlepass where I have maybe a 5 percent chance of landing it on a good day and a 60% chance of eating shit and letting go of the kite. I'm STILL gunna try it but will do so leashed to the 5th. kite lightly falls out of the sky, and I don't get dragged into the lawn mower 4 line lengths downwind of me that finally learned to kite upwind. Then I stand up and sort out the kite. I'd say 9/10 times it comes up no problems. (Just gotta keep tension on the 5th, 1/10 it gets a 5th tangle that I can unwind and maybe 1 in 50 I have to bring it to shore and straigten it out. On a frontline or rearline safeties I`ve tried, it`s almost always a walk back. odds get worse in the waves and deeper water where I can get pushed downwind faster than the kite and it`s harder to keep the 5th line tight. But C kites lose their appeal for me in the waves and I`d rather fly something that drifts a bit faster.
3) I crash a lot, I`ve tried lots, relaunching a C kite without a 5th really sucks in wind that requires anything bigger than a 10m. The faster you can relaunch a kite the less likely it`s gunna roll, the less likely you`re gunna have to go back to shore and straighten your lines. Eveyone advertises they`ll relaunch with the pull of a steering line but this is BS in anything other than high wind.
4) self landing. No beaners, No talking to beachgoers telling them to hold your CL (WTF! by the way!!!

) No newbie grabbing your trailing edge and sending your kite for a loop in gusty conditions.
Kite low to the ground, edge of the window, release to 5th give it a tug and kite dumps leading edge down. if this fails, (which it hasn`t on me yet) kite is TOTALLY depowered. So walk up the 5th.
5) safety, When you need your safety, you NEED YOUR SAFETY. Pull the pin kite is DEAD. Not 70, 80 or 90% depower. That 1 time in 6 years when without a visible front the wind gusts from 12 knots to 35 on that storm front that was SUPPOSED to miss you by 200 miles. Most manufacturers of non 5 lines JUST started to put the safety line through the bar...recently!!!! (like 2010) The first generation bows you had to leash OVER the bar to one of the front lines. so you either leashed suicide or risked catching one of your steering lines in the leash clamp.....you couldn`t even do a back roll without tangling your leash if leashed to the flagging line.....Uh WTF
So far I`m liking the North and Switch safety systems when rigged to a 5th. There are other effective ways to flag a kite but it, seemed the `dangerous` C kites already had the concept well sorted with the 5th line circa 2005 and have improved to deal with release under load and with sand contamination. I`m finally convinced the safety is going to work when I need it (so long as I pull it) and am reluctant to change at this point...