uhhhhhh Floyd uses the jab pretty well to the head and body......and Floyd is well fundamentally sound in defense and offense.....Roy was not fundamentally sound in Defense it was all based on his reflexes and ability to not be there when the punch was thrown.....case and point in both the Tarver Glenco KO's he was backing away to avoid the punch and couldnt get out fast enough.....Floyd is a far better boxing talent but RJJ is just a freak in his prime because of how athletic and unorthodox he was then father time caught him....now he has to try and rely on boxing talent and not just athletisim which is hard when you based your whole career on being just overall athleticly better and faster than everyone else.
JMM Winky Taylor Pavlik Wlad Quartey Kessler I could go on and on....its not as underused as you would think.
Most top fighters use an effective jab. The fighters who neglect it are not usually elite. Hatton was able to attain top 10 P4P status without much of a jab, otherwise, most use it as a range finder, a measuring stick, a weapon, to get inside, or to set up other combinations with it. Some fighters can get away without using a jab too often because of their reflexes.
The jab is my favourite punch next to the straight right. When done right, you can control a fight with a sharp and well timed jab. As for Mayweather, he used to use it a lot more in the lower weight classes, but to be fair he did use it a bit again recently to knock the wind out of Hatton when he jabbed to the body.
Why use it if you don't need it per say. By the way Floyd isn't Roy, he is much better technically and will age better if he chooses to