Inability to pay taxes means the whole economical system goes down.
No taxes = no economy. It's basic. A state, a government cannot function without taxes, they are essential for a countrys development.
And because you can't tax zerocoin, most probably governments will outlaw any legal transactions with zerocoins. So exchanges will not be able to trade these coins legally. They will still be used, but their value would be almost 0.
See my point? Which is also where bitcoin is heading if it can't be taxed. If bitcoin is to be universally adopted, it would have to be regulated.
Which kinda sucks. Don't get me wrong, I don't like the idea.