At my company our theory (and, I believe those of other airlines) are to prepare for a possible evacuation. Lights are off for takeoff and landing so that "if" the emergency lights were to come on it would be obvious to the passengers that they were on, and where the emergency exits are located. This also allows the Flight Atttendant to see outside the aircraft. In the event of an evacuation the FA would be able to evaluate the conditions outside the aircraft (fire, smoke, debrise) and make a decision which side of the aircraft to evacuate out of. Same reason the seatbacks need to be up (don't block the row behind you in in case of evac), and windowshades up (FA view outside in case of evac).
As far as electrical load goes, airliner electrical systems are designed to "shed" excess load if need be, ie engine/generator failure. My aircraft has five generators running during takeoff, and three of them could fail before there would be any shedding of load. We do normal takeoff's with all exterior lights on, radar on, beacon and strobes on, cockpit glowing with CRT's, and usually coffee brewing for the PAX in back! So electical load on the engine's during takeoff really isn't a consideration, now bleed air, that's another story...
Also, we do the vast majority of our takeoff's at a reduced power setting to increase engine life.
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