As an example of a case where setting the ratio low is advantageous:
We were testing a NAS system recently. Our tests use 54 systems writing to the NAS server. The default value of /proc/sys/vm/dirty_ratio was 40. We saw very bad performance when we applied a large write to the system. The vendors technical support noted that the writes where going to the NAS out of order and that because we had a large number of writes we where defeating the NASs cache forcing the out of order writes to become a read modify write cycle. By dropping the value to for example 2 we saw the NAS system perform.