I've got 5th lines rigged on all my bars now, but only as a relaunch aid. The main trigger in me fitting them was teaching my GF to kite, but I leave them on the bar when I fly too. It's nice to be able to roll the kite over in a second or so without having to swim. It's also nice to have a controllable way to depower and self-land the kite without having to toss it to the leash
My self-landing method now is:
1. Pull the 5th line until the kite stalls forward and falls down upside down.
2. Hold the 5th line in my teeth while I get my harness off etc.
3. Tie the 5th line to my board handle.
4. Put my harness on the board for a bit of extra weight.
5. Hand-over-hand up the 5th line and retrieve the kite.