Id assume a bot-net would go for large offices/companies, where people would be unaware of the extra pc usage.
Large offices/companies wont have very powerful GPU's if any at all.
It would be easier to organise a CPU botnet than a GPU botnet, because i can farm large amount of CPU power from "large" companies, but with GPU i would have to gather from individuals, which would be harder.
Botnets are more myth than reality in cryptocurrencies.
Good point
Id say CPU and GPU only is a myth also.
As mining becomes ever more profitable, market pressure will lead to faster CPU speeds and ultimately ASICs.