There are better things to worry about then mining botnets. Here are the simple reasons:
1. Most computers do not have a decent GPU that is needed for mining, so GPU mining botnet is not viable.
2. CPU mining is about 2 orders of magnitude slower than GPU, so the botnet with thousands of nodes would be less powerfull than most rigs of the casual miners.
3. Infection is easily detectable because of the spinning fans, thus modes would live very long.