No I don't think it's that common yet, but I find it rather useful.

The main purpose it cleanup. I'm from Germany and OSM is in parts still a hot mess from the early days. Adding this gives you a chance to search for traffic lights which may have not yet mapped crossings, as you can search for highway=traffic_light without =separate or =no.

Apart from that details like this can always be useful down the line, as this allows to determine easily which traffic lights are basically two separate objects vs traffic lights which are a single object for letting pedestrians cross. 