I went through the same experience when I started building my own wheels a few years ago. I wanted butted spokes, but my LBS gave me a whole "straight gauge spokes are stronger and the weight savings are negligible" argument. Turns out he was wrong on both accounts, and that he just didn't stock DB spokes. I can't figure out the why behind this, except that maybe the shop did this to keep their wheel build costs lower than other shops in town (high % of customers can't tell the difference).

You could find a copy of my wheelbuilding bible, or just trust the words of Sheldon:

"Double-butted spokes do more than save weight. The thick ends make them as strong in the highly-stressed areas as straight-gauge spokes of the same thickness, but the thinner middle sections make the spokes effectively more elastic, allowing them to stretch (temporarily) more than thicker spokes.

As a result, when the wheel is subjected to sharp localized stresses, the most heavily-stressed spokes can elongate enough to shift some of the stress to adjoining spokes. This is particularly desirable when the limiting factor is how much stress the rim can withstand without cracking around the spoke holes." Link

Scrod can probably give more insight on this one. I will say that the additional cost of doing a DB spoke build vs. straight gauge stacks up quickly depending on your spoke count.