I'm using a one-piece Stich. Bought it used and it fits my mission perfectly. 95% of my riding is commuting, so easy on and off over business casual is an absolute requirement. Most rainproof I'd need is an hour, if it leaked a little, that's not a big deal. Heated gear and a thin layer gets me below freezing with comfort. Open the vents, it's okay up to 75-80, IF the humidity is not too bad. Above 75-80, I use a Tourmaster mesh jacket and jeans. Not ideal, but I can't find a good set of riding pants or jeans that work for the business casual requirement.
If I rode just to ride, or toured, I can see how a jacket and pants combo may work better. Taking a one-piece on and off repeatedly would maybe get old.
I ride to work in the cool morning, so the Stich is OK, but the ride home I switch to the mesh jacket. Not ideal from an ATGATT perspective, so I'm considering some MotoPort mesh gear, or an R3 light.
RT