@readysetawesome After I jacked my shoulder, I went and checked out motorcycle jackets - even the lightest "summer" models with the mesh venting are just too heavy for where I live (at least, any of the jackets in my price range; it's possible something high-end would have been better). They might be OK while moving, but the minute you stopped at a light you'd want to die. That's going to push you to take risks so you can keep moving and never stop; run stop signs or stoplights without checking to REALLY make sure the coast is clear.
And if you don't want to wear your safety gear, you either won't wear it and then get hurt, or you just won't ride.
So what I made for myself feels like a reasonable compromise - light enough that I don't look at it and go "oh jeez I'm gonna sweat to death", easy to put on, and cheap. I pop it on with the rest of my kit and go.
Yeah, I already had elbow pads, knee pads, wristguards, and even padded snowboarding shorts that I wear under my shorts, to protect the tailbone (and of course a helmet). The shoulder is the one thing that doesn't seem to have readily-available lightweight protection options.
If I lived somewhere cooler, I'd def. go with a moto jacket because that's a nice quick one-step thing, but there's just no way here. I'd get heatstroke, or I wouldn't wear it, or I wouldn't ride, or I'd ride and take risks to never ever stop for anything.