Not a dumb question at all. Silnylon doesn't breath. The advantage is its weight and how small it packs down. I planned to keep it in my pack when rain was a small chance, if I "know" I'll be hiking in the rain for an extended period I use a Gore-Tex shell. But in all honesty, Gore-Tex doesn't breath all the well anyway, I still get sweaty wet if I'm exerting myself.
On my scale, the AGG jacket weighs 4.5 ounces.