If potential users of the currency believe that bitcoin is simply a way for botnet herders to convert spare cpu cycles into cash, they are unlikely to participate in the emergence of the economy. The current system could easily result in the perception that bitcoin is basically being used as a vector for the indirect monetization of the theft of electricity via compromised computer systems. I assume most people interested in bitcoin understand that self-interested individuals will obviously seek to exploit opportunities like this, and the design of a popular currency has to prioritize how a prospective user will evaluate its fairness.
There's no reason to think of it that way, anymore than the local barber is collecting hair to clone an army of super-men for world domination. You have to draw the line somewhere. If someone is going to steal electricity, targeting a 30 watt computer doesn't make a lot of sense.
I have seen the topic come up here a few times "what if someone seeks to exploit this?" and the answer is the same over and over, it has been designed from the ground up to counter this exact concern. If you want to "game" the system, you need a lot of CPU power. CPU power cost money. The system scales itself to the available CPU power it has so if you bought 10,000 EC2 sessions from Amazon and started a massive bitcoin generating farm, you would never make more money out of it than what you put into it for long because the system would self-adjust against it. If someone throughs a massive botnet at it, the same thing will happen. The system is set for self-feedback, so to try to steal from everyone is to try to steal from yourself at the same time.
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.
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.
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.