Dealing strictly in bitcoin would make it very difficult to sufficiently track a person's income. This is especially true for people who do a lot of "side" work as they could simply provide a different address to send payment to for each payment. A person could easily tumble their coins to hide the fact where they originally came from (a particular employer who could then trace them to the employee).
Currently it seems that, for many bitcoiners, just owning coins is all they want from life. But, in that distant future we are considering, that will be sort of passé: people will presumably want to
spend their bitcoins too.
Suppose you buy a fancy car with the bitcoins from those "side jobs". The tax detectives will know about the payment, and will link it to you, from the dealer and other sources. They check their databases and discover that the payment did not come from any of the addresses that you declared in your tax forms. What then?
Mixing won't help, quite the opposite. The tax office does not need to find out where your extra coins came from; it is you who will have to explain -- and prove -- to them that you paid the tax on that extra income.
Ditto for coins from "dirty" jobs -- if the tax guys suspect that your extra coins may be related to illegal activity, they will warn the police. Then it will be you, not the police, who will have to explain -- and prove -- that those coins came from legal activities.
Note that the government does not have to catch every single tax evader. They only need to catch and sternly punish enough of them to scare most of the other citizens into voluntary compliance.