An account with an exchange is the only realistic way to convert your BTC assets to an actual currency (i.e. something that is actually, not just theoretically, accepted in exchange for goods and services).
If mixed coins can't move through exchanges, they're worthless. If merchant acceptance of BTC increases, they could have some value, but still less than the more useful exchangeable coins.
I think you're right that blacklisting people from exchanges based on the coins they've tried to exchange is a bad idea, but you could much more simply just blacklist the coins.
And if you exchange these coins for other alt-coins on an anonymous exchange like shapeshift? And then exchange them back to different coins? How is that gonna work out for you?
If mixed coins can't move through exchanges, they're worthless. If merchant acceptance of BTC increases, they could have some value, but still less than the more useful exchangeable coins.
I think you're right that blacklisting people from exchanges based on the coins they've tried to exchange is a bad idea, but you could much more simply just blacklist the coins.