- Existing cartographic products are based on a fixed scale which is some round number. When a cartographer designs a map to look right at 1:100,000 or 1:25,000 or whatever, it does not make sense to rescale it to 1:136,495 or 1:34,124 or some other arbitrary rational number.
- Image products require a lossy transformation process to convert the raw (or base) image to a scale that matches the tile set. This transformation reduces image quality. When someone goes through the effort of collecting imagery at 0.5m (or better), it is a shame for that content to be degraded unnecessarily before reaching the end user.
- When people look at imagery, they are not interested in looking at the second-highest zoom level. It just doesn't happen - they either zoom out enough to get their bearings or they zoom in as far as they can. If your tile set is based on powers-of-two, your second-highest zoom level adds about 25% of waste.
Alas, we compromised by relegating support for non-powers-of-two zoom levels to an extension. I will be interested to see what happens as organizations commit to distributing raster products (especially imagery products) via GeoPackage and the ramifications of the powers-of-two approach become more apparent.
 These values are for the equator; the values at ±30° latitude are about 3.5% lower.