"Note: You do not have to use the si- summary index search commands if you are proficient with the "old-school" way of creating summary-index-populating searches. If you create summary indexes using those methods and they work for you there's no need to update them. In fact, they may be more efficient: there are performance impacts related to the use of the si- commands, because they create slightly larger indexes than the "manual" method does."
Hmm. It's not always true that si generates large results. For example, for computing medians and percentiles, si will likely be considerably more efficient than whatever you come up with manually. However, there will also be cases where you can't use si commands to summarize and retrieve the data as you need.