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.
When I used bnet to crack some rar password, after few days of work I noticed about 20% drop in quantity that never really recovered. And it was not worth the efforts, since the rar file contained basically crap.
Bandwidth, some bursts of activity - ok, this is not noticed by most losers. 100% CPU load might make machines too unresponsive to alert some seek for help. And if the loser is joining other networks as well, the computer in most cases is like Pentium 2 struggling with Windows Vista.