I'm looking at ways to reduce the amount of power used by my laptop, and one of the things I came across was enabling laptop-mode. Apparently this is enabled when I'm not on AC power, but I'd like to save power when running on AC as well. Is there a reason why this isn't default?
I assume you are asking about the special laptop-mode in the kernel, that spins down the hard drive when not used, and writes down data in "bursts".
I can see three reasons:
1) There apparently are issues with Ubuntu and laptop-mode. Computer hangs, and thus it is disabled by default.
2) From the laptop-mode's FAQ we can read that desktop hard drives aren't that reliable for spin downs-ups as laptop hard drives.
3) This is a power saving feature, and nothing else. In chance of a computer crash, you will lose all data not written down to the hard disk. And since the hard disk is suspended for periods of time, you will lose more data with this enabled than if disabled.