It's more than just that, especially if your using a bow for instance (since they generally have much larger crit multipliers and sometimes ranges.
So say you have it 3 times for +6 base ranged damage. If like above you have 400 ranged power, that multiplies your ranged damage by 5x.....so it auto adds 30 damage a pop. That may not seem like alot when you are hitting for over 1k, but that is before you crit, which could boost that to 150+
Also every little bit helps, every 2 dex is just +1 damage, so it is worth 12 dex (although with a longbow and manyshot, it's just 8 dex...but still nice.
It won't break the game, you won't blow without it, but if you know you are gonna be using a bow, you would be insane not to pick up 3 ranger past lives. Also remember they work for the whole trip through the leveling process. It's not just about how much it'll add at 34, but how much it'll add at 1-20 as well, and 6 extra base damage is pretty huge throughout that process, especially with AoE shots now.