> Any moment the value of it can drop to zero
As opposed to... what?
> someone might walk away with the contents of your wallet
Completely depends on how much you care about securing your wallet. I wouldn't say this is a common occurrence at all.
> and that someone might be your exchange.
An exchange is where you go to trade currencies, why are you keeping your money there? And how is that a problem of the currency?
And isn't it obvious it can't satisfy a micropayment niche because ... it's a scam? Any moment the value of it can drop to zero, someone might walk away with the contents of your wallet -- and that someone might be your exchange. All of these had examples.