If people only had one generation's worth of memories in their brains, they would be ignorant to everything learned in the past. For example, humanity's problems, bad habits, and quarrels would probably repeat themselves because each generation would have to learn those lessons all over again. No one would remember the successes that the previous generation achieved, either; and, the next generation might miss out on the benefits and lessons learned from those as well....
If people only had one generation's worth of memories in their brains, they would be ignorant to everything learned in the past. For example, humanity's problems, bad habits, and quarrels would probably repeat themselves because each generation would have to learn those lessons all over again. No one would remember the successes that the previous generation achieved, either; and, the next generation might miss out on the benefits and lessons learned from those as well. Jonas learns about some of the effects of this issue in his world when the Giver gives him a memory about Christmas time in chapter 16. Jonas learns about multiple generations, such as parents and grandparents. He also learns about family and love which are both taken away from Jonas's community because people aren't allowed to stay in family units after the children are grown up. Because of this memory, Jonas feels sad that he doesn't have a multi-generational family.
Another example of having only one generation's worth of memories are seen in Jonas's household. Jonas's mother and father help him to prepare for the ceremony for twelve-year-olds by drawing from their own experiences in chapter 2. For example, Jonas's father says the following:
"So I watched and cheered when my sister, Katya, became a Nine and removed her hair ribbons and got her bicycle. . . And finally, at the end of the second day, which seemed to go on forever, it was my turn. It was the Ceremony of Twelve" (14).
Here, Jonas's father can help his son wrestle with his apprehension for receiving an assignment by relating to a similar circumstance he had when he was twelve, but he cannot give him any other advice. Nor can he draw from history for stories about other young children who overcame fear or apprehension. There's just a lack of more developed information from which to draw upon to apply to their current lives.
Finally, as Jonas discovers more about history and intelligence from the memories, he gains traits that the Chief Elder said he would on the day he was assigned as the Receiver in chapter 8: intelligence, integrity, courage, wisdom, and the capacity to see beyond (62-63). The community with one generation's worth of memories lacks most, if not all, of these traits. Life becomes droll and less purposeful. Without a lengthy history to draw from, much of the joys of life are lost.