I have seen so many older trucks with mounted bumper lights where either they are rusting or corroding around the light, the light is rusted or corroded, or both. The last thing I want to do is have a bumper replaced or re-sprayed. With the under mounted option on the tow bar, they can be easily replaced.
In addition, I can adjust the angle to shine down the dark, cold, and slippery ramps when I am launching my boat. With a bumper mounted option, the light would just shine off into the sky when I launch the boat.
Multiple reasons why a bumper flush mount won't work for me.