MMA at its most fundamental is a display of what martial skills are most effective against an opponent when forced to fight within specific parameters. It's also an exhibition of which of two fighters in a given match is ultimately a better practitioner of these skills and able to tie them together into an effective whole. If we are lucky it’s also another piece of the elusive puzzle that asks who is the overall best at this within a given division in the sport.
BJJ, Wrestling, Boxing, MT, Judo, etc... Are all martial skills that have elements that can be incorporated to answer this question? Development of these skills is a natural process of their utilization and central to the whole point of the sport. Weight training, dieting, endurance training, etc. are all natural means of honing the body to best utilize these skills in the safest most effective manner possible to the individual. The most important thing to point out is that these are all progressive processes which either maintain or develop peak efficiency
Weight cutting through dehydration is a process whereby the body is put in an unnatural and unhealthy state for a limited duration and which requires the athlete to have to try to correct as much as possible prior to their fight in order to not only provide a more natural performance, but also return a measure of safety to the activity. Those that are unable to sufficiently recover from this harmful unnatural state on average are unable to perform with the peak efficiency all the months of training prior is supposed to allow for. Overall we see subpar exhibitions of skill and for shorter durations. This means less value on investment for those that actually pay to attend or pay for the PPV. It also means the important question of who is actually better at MMA-style fighting is less likely to be answered and it’s more likely that a fighter may sustain a concussion or dehydration related injury such as being more susceptible to being cut because the skin is now less elastic and less able to sustain crushing or tearing force.
The weigh in process as its currently practiced in MMA does nothing to truly level the playing field as it was intended. It allows an activity that harms the live application of the sport by keeping many athletes from showcasing the very highest levels of martial skill and athleticism, levels they could easily maintain for three rounds, if not five, otherwise.
It allows a "dummying down" element to be introduced into the contest whereby an athlete can simply look to cram in a strength advantage through a procedural loophole to offset skill disadvantage rather than force an athlete to rigorously apply their time to technique development or to develop faster better situational awareness and strategic thinking while in the octagon.