Modifying your robot during a competition has become one of the staples of modern robot combat, and disallowing the practice is next to impossible on account of being able to distinguish between a repair and a modification (eg. a side panel gets ripped off, is replaced with a side panel made of a better material to counter their next opponent. Is that a modification or a repair?). I also feel that it adds another layer to the design and ingenuity aspects of many builders, and can give chances to machines that would otherwise struggle helplessly against their opponents. Not to mention the machines that have interchangeable weapons built into their design. Examples of which include Tornado from the classic series, Ghost Raptor in Battlebots S1, Bombshell in S2, Big Nipper, and hell, those are just the prominent ones. I'd estimate that half of the machines you see fighting have interchangeable weapons. Removing them would just be impossible.