However, one thing is quite funny: the hacker or bot relies on solo mining. Not a good source of income. 
But, couldn't it be a gateway to the person's home network?
You may not earn anything from mining, but you may be able to earn money from information you obtain from other devices on the network.
Well, I don't know specifically the problem that could be generated by this backdoor. But, it is never a positive thing to have a device with a backdoor connected to our network.