I haven't experienced any dramatic shift from the backlighting, they are using LEDs which don't draw very much power. I'm getting 6.5-7.5 hours on battery doing normal knowledge worker stuff (Office, Web, Reading, collaboration, etc.)
I wondered about this too. The sole reason I sold my SP1 for an SP2 was to increase battery life. I choose to keep my type keyboard 1 thinking I'd rather have what little juice goes to the backlighting go to extra run time instead...but I really think the backlighting is cool.
Keep in mind that you can turn off backlighting completely on the new Touch/Type covers. Especially for the Touch 2 where the biggest advantage is the increased sensitivity, even if you don't care for backlighting, you can get the Touch 2 just for that reason and disable the light. Or if you prefer colors on the Type, same thing. Or if you hate the felt backing. But finding the first gens will be cheaper overall.