This topic comes up peripherally in lots of other threads like the ones about compact crank sets. For whatever reason cassettes with 13t smallest cogs would be a tremendous boon to many riders running 53/39 chain rings. In 2 X 10 speed a 13, 14, 15, 16, 17, 18, 19, 21, 23, 26 would be a great standard cassette with no holes up to 19t. In 2 X 11 speed you could just add a 30 tooth largest cog and still have the tight spacing for most riding along with a very reasonable bail out gear. These cassettes would also work great with 52/36 chain ring combinations, giving a 4.0 gear ratio like was so common back in the 1980s as well as the tight spacing throughout most of the cog range.
How would it hurt Shimano, Campagnolo, and SRAM to offer these combinations everywhere up and down their product lines? It doesn't detract from the race pedigree of the upper end group sets to have cassettes available that are more suited to how the common folk ride. If you pedal a low cadence downhill and don't like to spin out, no problem, you could still have your 11t top gear. But for many of us the 13t top cog would be just fine. And filling in the 16 and 18t holes would be much appreciated as would getting the 30t low end with 11 speed without giving up anything in the middle of the range. Heck, even a 14t high end would suit some folks.
So how do we make this happen?