I don't know what angle you are coming from, but by far and large most uses of UTF-16 are entirely due to legacy reasons – APIs and languages made before Unicod

I don't know what angle you are coming from, but by far and large most uses of UTF-16 are entirely due to legacy reasons – APIs and languages made before Unicode extended beyond what fits in 16 bits. Back then, the trade-off made sense, and was popular too, because variable-sized encodings were much more rare. These days UTF-16 really is the worst of both worlds, but we are stuck with what we have.