I don't know... You make about 0.1 cents per hour mining with a in the wild CPU, that's a lot of emails that could be sent out. Even if spam has a conversion rate of 1 in 10 million it is probably more worth it.
The main danger is that the target notices that their machine is using 100% CPU and removes the bot. It is easier to notice CPU load (heat, fan noise, prominent position in Task Manager) than network load.