Botnets in my opinion is not effective miners. They mostly is CPU based and the 100% load might alert few compromised losers to reinstall computers. All other is true.
thats not how it works. they mostly rely on the fact that the user will not notice any changes (increased load, saturated bandwidth) to their computer -- typically because of user ignorance. if the first thought was to reinstall, botnets would not be in as big of use as they are today.