I have to second the statement that breathable shells do work. Softshells, I don't know much about yet. "hard" shells aren't magic, and they won't work in all conditions, but they're better than garbage bags, in most cases. There are a million factors that change how well they work, but no breathable shell by itself will suck the sweat off of you.

First, I think it's important to remember that water has to be in vapor form before it can go through the membrane (I don't remember the exact reason, but it the size of the clusters of molecules needs to be smaller than the clusters in liquid water). Even so, these fabrics are passive and won't actively pull the vapor through. If it's humid outside, the vapor will only diffuse through the membrane until the humidity inside the shell matches the outside humidity (true osmosis) and it isn't instant. So, there will be some "drag" on the system. If you're sweating faster than will pass through the membrane, then it may condense and decrease the diffusion rate.

Wearing synthetic fabrics underneath a breathable shell does help keep your skin dry, by pulling the moisture into the fabric, where your body heat can more easily convert it to vapor. Cotton holds the moisture next to you, which can be a little nicer in a hot, breezy climate, but not inside a sealed-up armpit.

If it's humid outside the shell, it'll be humid inside. Venting is great, to dry sweat and prevent it in the first place. Some membranes are more porous than others, but none of them will pull liquids through.

If you're gonna sweat buckets somewhere humid; vent or just enjoy the shower. If you're not; try a breathable, but leave the cotton behind or that'll just "seal in the juices."