No backlight means each pixel has to work harder the more you increase the brightness. OLEDs displays have a shorter shelf-life than LCDs. Also, the brighter anything is the more energy is used...
Well after owning a Note II for two days, I have to say I'm pretty impressed with the battery life and think a Note II user doesn't really need to worry about it, regardless of brightness (I have mine manually set to about 2/3 max).
My normal usage on my S3 included a couple hours or streaming music, checking work and personal email throughout the day, checking FB and surfing occasionally throughout the day, even an occasional video. After about 2 hours of screen on time - by late afternoon/early evening, my S3 would be down to <20%.
After a full day of usage (midnight now) + playing around with my new phone, with 3+ hours of screen on time, battery life is still 45%.
In comparison, my old iPhone 4S would normally get me to about after dinner and also be <20%.
And I suppose if I planned on using this Note II in 2017/2018, I'd have to worry about screen longevity. Luckily I plan on upgrading to the newest and greatest in <2 years.