The dual drives are most commonly seen on smaller wheeled bicycles and folding bicycles where the reduction gearing in the hub offsets the gearing differential you get with a smaller wheel size and simplifies things by removing a front derailleur... for standard bicycles an 8 speed IGH is the sweet spot for commuting and all around riding on a non performance oriented bike.

My friend has a Bike Friday with a dual drive but retained the double crankset which he shifts manually (no front derailleur)... while touring in Europe he found that the regular setup and gearing was fine for most conditions but appreciated having that extra granny gear up front when the roads pointed straight up.

His bike has 54 speeds although there is a fair bit of gearing overlap...