The reason e-bikes do not have regenerative braking is because bikes don't have enough momentum to make it practical, especially compared to the efficiency loss which comes not from "cogging" but the reduction gearset.
Also, mid-drive bikes are increasingly popular because wheel motors have a bunch of disadvantages. They're currently more expensive, but that may eventually change with time. Production cost reductions may be offset by the fetish manufactures are showing for proprietary mountings, and Bosch is DRMing the fuck out of their systems.
I have a Bion-X with regenerative braking. I live in Seattle. It is extremely useful in the hills; my house is up a hill, if I start fully charged it's really annoying riding the brakes all the way down, but when not it's really nice glide.
Also, mid-drive bikes are increasingly popular because wheel motors have a bunch of disadvantages. They're currently more expensive, but that may eventually change with time. Production cost reductions may be offset by the fetish manufactures are showing for proprietary mountings, and Bosch is DRMing the fuck out of their systems.