I've been thinking about it some more and realised there is another aspect to the functionality. A couple of other points are...

Without the reinforcement tape, the leech tension would still be transfered to the head along that bypass route more than over the top of the head, but the sail material would stretch more.

This tension along the bypass route also holds the batten relatively straight where they cross. Imagine the bow string analogy (that's a good term for it) with an extra bow string going along the bypass route and fixed to the batten where they cross. The batten will be held there and bend either side of it. You can see this if you capsize your boat on land and crank the downhaul on till the top is completely flat and the batten starts to behave funny, it may even invert.