LoVel - you had me worried there for a moment: "I have gone without the pants before..."
I've got to admit that sometimes just popping around town, I'll do without the jacket. Boots, jeans, t-shirt, gloves and helmet yeah, but sometimes in 90 or 100 degree heat, I've left the jacket behind.
Out on the open road though, even in the heat, I've got that mesh jacket on. Amazingly cool in the heat as long as the wind is flowing through it. I couldn't believe the difference between the mesh jacket and my old non-vented, leather jacket... The heavy leather jacket will still get the nod for cool weather riding though.
So yeah, I cheat in the heat, on casual in-town rides. Cost could well be some road rash someday... Hope not...