I can tell what gear I'm in by the speed and revs, but I only have five to pick from, and really, when moving, I'm using four of them, and at moderate speeds on backroads, I'm using three of them.
If I was driving a WRC car with gawd-knows how many gears and and engine that revs like it has an anti-gravity flywheel, and a navigator barking crap at me, I might want that indicator. For sure a shift light!