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The only thing I use that causes the fans on my Core 2 Duo MacBook Air to kick in is Flash, so I would definitely consider a slower CPU.

Also, note that the savings/benefits can be in ways other than more battery life e.g. price, fanless & sealed cpu, etc.

How are they mutually exclusive products?

Does it still have plenty of battery life when you're doing something besides commenting on Hacker News? Perhaps more importantly, how fast do you need a single instruction stream to be? I'm writing this on my own three-year-old MacBook Pro, but I know that its battery life is mostly contingent on the CPU remaining idle 99% of the time. For any multi-tasking workload, or even for single-task workloads if the software is written correctly, a more efficient CPU architecture with many smaller cores would offer both greater responsiveness and longer battery life even during my most computationally intensive moments. Maybe you're one of the 1% who really need higher single-thread performance, and not just because of crappy multi-core-naive software, but the market is driven by the other 99%.