But there are two trains of thought with Ich. There is some evidence that Ich is present in most aquariums at small enough numbers where they can't affect the fish. Then when stress weakens the immune system of the fish (sudden temp. change, etc) the Ich is able to gain a hold on the fish. The other train of thought is the one you mentioned, where Ich can only be introduced and doesn't exist in small numbers in the aquarium. Last I heard, the fishkeeping community is still undecided on which is correct.
I agree that for most people simply using proper QT procedures, avoiding stressors, and observing the fish carefully will be enough to prevent Ich. But I see it as kind of a back-up plan I guess. When your fish are very precious, one can never be too careful