Topping lift
Another penny worth of input, but yes, the "soft" TL is NOT generally a sail shape device, and was NEVER intended for that purpose. As has been noted, if the sail is not quite right, or the boom is unusually heavy, it CAN be used to slightly raise the boom, and allow the leech to twist a little, or "relax" in lighter air, which allows air to move more freely off the back of the sail (remember the whole idea is to move the highest volume of air accross the sailplan-from luff to leech. The "job" of a sail is not done unless and until the air has "left the building" (the leech).
This would only be done in very light air, and the danger for many is in not having it long enough (or not easing it enough) so that instead of trimming against the actual sail, you end up pulling against the topping lift-which does no good.
The best solution is always the solid vang or kicker because you can adjust the leech tension freely and without limit, and you avoid the windage and nuisance of the TL. They were never intended to be used to shape sails though, that is an unplanned option to deal with less than ideal sails or excessively heavy booms.
Good times,
Seth