wire luffs
The use of a flexible wire in the luff of a headsail is a common practice, often paired with hanks or some other method of attaching to the forestay. I have removed the wire from some sails, and modified others to still work with the wire. The reason is that the wire will never shrink, but sail material does over time. Just like the problems encountered with bolt ropes, and the difference in shrink rate between the rope material and the sail material, headsails with luff wires with some years on them will often suffer shape problems. On a sail that has shrunk in relation to a wire luff, you can see a lot of slack in the luff wire when the headsail is tensioned by the halyard, and I mean a lot....
This is not about sail strength, as many sails are made without wire in the luff. Sails that I have removed the wire from look good and are strong enough afterward. If fact, I haven't got a good reason to put a wire luff in a sail. Many sails from performance to cruising do not have a wire luff.
I am sure that if you are buying a new sail, and you insist on a wire in the luff, the loft will oblige you. It might be that the wire will fit into a foil if you plan on installing roller furling, but then again it might be too small. You can also kink the wire in storage, and your sail will not store as compactly with a wire luff.
I have done a mod to wire luff sails where I install a strap at the bottom of the sail, that allows adjustment of the luff tension, after the halyard pulls the wire luff tight. So now what was the question?