Let's say, for the purposes of this discussion, that most regular people have 2 CPUs working for them. Maybe they're high end, maybe low end, but let's just look at the number of CPUs at this point. Regular miner has his 2 CPUs, now botnet miner likely has tens or hundreds of thousands. Really good bot herders can even break the 1M bots mark. You can see that even if we consider the number of bots the botner miner has as half of the total - to account for shut down PCs, loss of bots, internet connection issues on the bots, etc.- and even to tip it more in the home user's favor, let's say he has 2 of the latest i7 Extreme Editions, or hell, even high end Xeons. Even if those bots are all rocking low-end Pentiums and Semprons, the botnet miner will kill the average users by a LARGE margin.
You misunderstand stand the point of egalitarian mining. Yes, someone with more CPUs will mine more than someone with fewer CPUs. That is obvious. But as long as it stays linear the return on investment is mostly constant, in fact the PC user has an advantage that is very similar to a botnet owner: his PC is already paid for, he didn't buy it for mining, so his capital cost of mining is essentially zero.
You can't expect to be miner extraordinaire with a PC but with linear scaling, you get your share. You can get a few coins out of it every now and then, maybe make a little bit over the cost of electricity (which only costs around 30c/day). At this scale its a hobby or even less. But there is at this point no reason to turn off the wallet miner, you might as well accept the free coins.
It is only once the relationship becomes non-linear (where buying specific GPUs or worse ASICs means high capital costs but massively higher hash rates than regular PCs) that this sort of casual mining is useless and it does make sense to turn off the wallet miner, and then everything becomes concentrated.
BTW, GPUs are definitely scalable, though not necessarily the way hobbyists do it.. Dga did his nv GPU mining using AWS GPU instances. Hundreds of them I think.
As an aside, this is why ASICs are awful: They are EXTREMELY expensive compared to GPUs, locking out the average person, they entail greater risk, as they cannot be resold if their mining usefulness ends before you thought it would, and they are plug and play, so you can hire unskilled labor to keep them running - and you'll need less labor, because they are far more stable.
Yes, that's right. ASICs are at one extreme, CPUs are at the other. GPUs are somewhere in between. If CPU mining manages to survive on this coin we will get something that is pretty much the opposite of what ASICs did. We'll see if that happens.