On the one hand the state would know about every transaction made, on the other hand the state would hold immense power over its citizens by being able to flip a switch and shut certain persons out of this system. (by freezing their accounts)
And then there's another aspect: In a cashless society only the ones that have jobs or can afford to incorporate a business could earn money. All the others would have to barter with smuggled cigarettes or vodka and even then they'd be unable to pay their electricity bill with whatever they've earned this way.
In tough economic times there are many people that cannot get a regular job nor can afford to incorporate a business, and working illegally on construction sites or something like that has traditionally been a way for them and their families to survive and work themselves out of the gutter.
If you take that away from them, they'll have to fully submit to the social system and I honestly doubt that this is a better way.
Our western societies are slowly turning in a modern version of the Soviet Union, where the state has even more control over its population and its thoughts than the Soviet Union ever had.
At least the Soviet Union had the decency to grant the masses that stayed clear of politics a safe life without a care in the world.
Here in the West they are not even willing to give us that for giving our governments absolute control. The masses at the bottom have to struggle for mere survival.