Here is what I know population to effect:
More population means more cash from economic based planets (Anyone know the formula?)
More population means more cannon fodder for wars (Troop ships)
Too much popluation without enough stims means unhappy population
More population means more votes in the United Planets
Now... what else does a high population get me? What I'd specifically like to know is if there is any point to cultivating a high population on a research pure planet?