CPU mining isn't very successful because of this.
The coins are never really worth much because a hacker would start cpu mining with their zombie network of 10,000 infected computers. They'd dump all those coins on the market and drive the price down to .00000001
Even if a hacker or a group of hackers was able to start mining with 10.000 infected computers they would not generate more coins than a few pools whit a lot of GPU-miners already do today.
I think that the solution that can scare off both hackers and pools is a reward system where rewards go down as soon as the network hash rate goes up.
Sometimes difficulty stays high for a long time since a high hashrate (attack) has been deployed and those who mine continuously get screwed..
One solution that some coin used/s I forgot the coin was to use a high ram requirement for cpu mining. They needed like 17Gigs of ram or some pretty rediculous amount to combat botnet-mining..
But in the end of the day, if an economic incentive is there, someone will create a gpumining program or asics anyway. If one wants to mine with a cpu effectively, one has to mine early on or just splash some cash and buy those gpus, it´ll be worth it anyhow , get it.