GPUs makes the network very botnet resistant. Bitcoin likely would have been destroyed by botnets already (if out of spite or to simply see if they could) had it not been for the rise of specialized (i.e. GPU) mining.
This idea that specialized mining may defend the Bitcoin network from botnets might have merit.
I wonder if it might be possible to have the best of both worlds - where specialist mining makes commercial sense and casual CPU miners can also convert electricity to cryptocurrency at a rate that isn't prohibitive.
I'd guess you'd need to see a ratio of about approximately - 1.5:1 (Efficiency on Specialist Hardward:General CPU) to have both co-exist.