My sail has a strip of what looks like a kevlar uni tape(or carbon, it's been awhile since I've looked at it) in the same position as the top diagonal strip in the photo.

I assume that because the leech tension can't be transferred around the very top of square heads, the tension will pull along those lines. If extra tensile reinforcement (such as is already in the leech) wasn't there, it would stretch the un-reinforced material.