I agree that having a written language in no way makes for a "better" culture than those that don't. The opposite, in fact, might well be true. I don't know how one could really compare all the written language vs oral cultures to determine that though, especially given that there aren't many of the oral ones left.
But one possible "benefit" I could think of would be the elimination of the "chinese whisper" effect. Oral traditions could be gradually changed over time and with nothing to refer back to you wouldn't know it. When there are written records, you could go back and compare the earlier versions with the newer, and so on. (Of course people could just then burn the records, a la "fahrenheit 451" or "1984"... but that's the possible advantage I can think of off the top of my head.