David Snyder says: "I fail to see how a 15g spoke in a slightly larger hole creates any additional cyclical stress on the bend."
It may not make a difference. What I have observed is that spokes break at the elbow, and far more often with straight gauge spokes than with butted spokes.
DS "Less windup than with butted spokes makes for a faster build. There's their lower price."
When striving to create the highest quality product for a customer, I don't consider the marginal time saving or the slight cost saving worth the reduction in quality.
DS: "these also seem to have a proper amount of elasticity for a lot of different applications."
Butted spokes, 14-15-14 gauge, have the same elesticity in the center section, which is not stressed by other forces.
DS: "Their added bending flexibility vs 14G spokes also makes building wheels a little easier, as does a freer fit through the spoke holes."
Again, easier on the builder is not necessarily better for the customer.
In short, David does present a good argument that straight gauge spokes are fine in a production environment where lower cost for parts and labor is one of the considerations.
However, like the carefully handbuilt frames we all admire, carefully handbuilt wheels using the best available parts and allowing ample time to properly use those parts are just a nicer product.
Earle "I'm willing to spend the time if the customer is willing to spend the money" Young Madison, Wisc. Offering expert wheelbuilding services for classic and modern bikes