Were cabin lights really consuming that much power back in 2006?
Thunderstorms normally mean icing conditions, which necessitates anti-ice, both through bleed air and electrical systems.

I can see this going a few ways:


Pilot is (for instance) at high altitude and really does need the additional bit of power to cross above the thunderstorm.
Pilot has flown aircraft type where this is a legitimate problem and figures 'why not?' for safety sake.
Could be something about motion sickness in the storm, since closing your eyes can be an effective countermeasure. Perhaps dimming the lights goes along the same lines.