Would the weight of these things actually be enough to throw off the balance? They don't weigh more than a dead cockroach or speck of mud or ice, which occur on wheels regularly.
I've got two reds and two blues. I'd never use them on my car, I don't want the ricer image (it's a Honda with alloy wheels) but they'd be great for bike safety.
They are also good motion detectors... fake car alarm devices perhaps?