The theory is, it can boost your battery life, now Nokia's default kernel is 250-600MHz using 1.075-1.350V, if you used the ideal profile, it will be 500-850MHz using 0.975-1.200V, so actually, since you are using lower voltages, it will help boost your battery, not to mention since your device will be more responsive, it will do most of its processes faster so it will take less time doing them thus saving energy.

Ofc, you can go commandos with the battery saving, set the profile to XLV and enable 125Mhz (which uses 0.850), more on that stuff here:

http://wiki.maemo.org/Kernel_Power
http://wiki.maemo.org/Overclocking

Enjoy