T-boning is when one team is pushing against the side of another robot, creating a T shape if both robots are long or both robots are wide.
Sometimes hexagonal bumpers help so that a team can only push their bumpers against a component angle of your side bumpers, allowing you to get out of a T-bone. Depending on how the team T-boning you orients themselves, hexagonal bumpers may not work.
Watch defense played on 971 in this match. Occasionally when a robot makes contact to the side of their robot, they can roll out quickly. Occasionally they can't.
https://youtu.be/G07Ci0VcUjs?t=40s
It certainly can't hurt to make your bumpers hexagonal, but you should ask yourself two questions before you decide to do so.
Is my team a major target of defense?
Is this the most effective use of my resources in handling defenders?