Keep in mind that as a zombie computer becomes more "obvious"- computer is slower, fan runs at 100% all the time, etc, the more likely that the malware will be noticed and removed.
I was mining Litecoins this morning and even though my computer was relatively quiet (mining with GPU) compared to fans at 100%, etc, it was still painfully slow. When I hovered over a link it would take a second to change colors. Literally a second. The user would probably notice that too. Though it wasn't laggy when I was mining Bitcoins so...who knows.