Consider two systems, the first one is 90% reliable and the second one is 99%. The wrong to compare them is to compare the reliability and conclude that the second one is 9% (or 10% if you take 90% as the base) better. The right way to compare them is to compare the unreliability and conclude that the first system fails in 10% of the cases while the second one fails only in 1% and hence, is 10X more error-prone than the second. The reliability comparison is a vanity matrix while the unreliability comparison not only demonstrates the user perception (“The user saw then crashes in past one hour” vs “The user saw one crash in past one hour”) drastically but also shows the effort that goes into making the system more reliable.
Note: While I am taking reliability as an example of the metrics, the argument can be made for similar metrics.