Plain gauge aren't necessarily weaker but they build a less resilient wheel. Double butting (or, more correctly, swaging) makes the material in the middle of the spoke thinner, which makes it more flexible ("stretchy!"). This allows the middle of the spoke to expand and contract more under varying forces, which prevents those forces being transferred to the spoke elbow or to the hub flange.
Spokes commonly break at the elbow, mainly because that's where they end up absorbing force if the rest of the spoke doesn't. I've built hundreds of wheels and IME double butted spokes build a stronger, more resilient wheel. My recommended spokes for really heavy riders would be Sapim Force – 2.3mm at the elbow, 1.8 in the body and 2.0 at the thread – they're thicker diameter at the elbow to be even stronger.
There's all sorts of technical bumf about this online, Sheldon Brown wrote loads, Jobst Brandt has covered lots of the physics stuff in his book "The Bicycle Wheel" and Roger Musson's written some very helful stuff too.
HTH.
: P