I think you are rationalizing away a serious issue. Botnets exist, and the owners of botnets seek to monetize them. Bitcoin minting is an available strategy. Saying "If someone is going to steal electricity, targeting a 30 watt computer doesn't make a lot of sense" is a complete non sequitir. Being in control of a botnet means YOU ALREADY HAVE STOLEN the electricity, the question is - how are you going to make use of it? It is very simple economics that bitcoin production is roughly proportional to energy input, and that therefore the most efficient producers will be those who are able to obtain that electricity at zero cost.
A botnet is a stolen computer already, the concern people have is that it will take over bitcoin in some way. What you are talking about is what we already know. They use the stolen CPU time to generate BTC and then sell it on the market. It's stolen CPU time, we have no control of that. What we do have control over is how much damage it can do to others that are running honest clients.
In making an analysis of an economic system, you obviously proceed by determining how self-interested actors will behave in that system. I am completely unconvinced by hand-waving arguments that try to claim people will not behave in selfish and unethical ways, if they see an opportunity for profit. We already KNOW that people are throwing a lot of computational resources into minting bitcoins, the steadily increasing difficulty is exactly equivalent to a steadily increasing computational cost of the system! The more energy invested in the production of the same quantity of coins, the harder bitcoin has to work to deliver value added on that cost.
The system was designed with that in mind. That's why it uses a formula that forces anyone that participates in the protocol to behave or else they are ignored. So if anyone is going to game the system, they have to do it by the rules that everyone else is following. Right now, the only rule to game is CPU time. It's either expensive, donated, or stolen.
Again, an analysis of the behavior of self-interested actors is instructive. The most energy-efficient scenario for bitcoin generation in the current regime would be if there was ONE NETBOOK that generated all the coins - the rate of coin production is fixed, so you'd still have the same amount of coins entering circulation. The person who ran that netbook would obviously be in a position to charge a high "rent" from the population that needed bitcoins for their transactions. An examination of this limit case makes it clear that bitcoin minters have a strong incentive to encourage use and circulation of the currency while discouraging additional minting nodes from coming online. This is not any kind of criticism of anyone's ethics - it is simply an examination of how the game is set up, and what strategies players will adopt.
As an overall point, I also do not agree with the idea that the very high computational burden of coin generation is in fact a necessity of the current system. As I understand it, currency creation is fundamentally metered by TIME - and if that is the fundamental controlling variable, what is the need for everyone to "roll as many dice as posible" within that given time period? The "chain of proof" for coin ownership and transactions doesn't depend on the method for spawning coins.
That's the problem with having one PC generate all the coin, that one PC is a central authority of it and can price it anyway he/she likes. To de-centralize it means you need some way to force everyone to play by the rules or else one person will try to bend the rules.
I respect that you don't agree with how it works, so I offer up that we be shown a better way that we can't all punch holes in. It's open source software, if there is a better way, it can be used. Currently the rules are why they are because the entire network is designed with "trust no one" but instead "trust everyone collectively".