For me it's about comfort. I have been a PC gamer all my life, I'm comfortable with a mouse, gamepads give me cramps and are simply not ideal for my go-to genre - shooters.
Really it's about using the right tool for the job. I wouldn't squat in running shoes or jog in weightlifting shoes, and I wouldn't play a shooter with sticks or a fighting game with a keyboard when an arcade stick is preferable and doesn't negatively affect my performance.
As for the so called 'even playing field', fact is there is no such thing. The closest you're gonna get is organized tournaments with uniform hardware, but even those aren't completely even, since there are differences even among the experienced players who participate in those.
Look at sports, top athletes pretty much all train the same, they all train hard, but not everyone is a gold medalist, a top scoring footballer, or Michael Jordan.
Even among controller users no one is equal. You have the casuals, and the MLG tryhards who are better at aiming with thumbsticks, and are probably using some mod for their controllers to improve their performance.
I would advise the people crying about their poor performance and blaming it on M+K players to focus on self improvement instead of looking for scapegoats.
I'm sure if you gave them all adapters they'd still play poorly and then blame it on something else.
As for citing previous tests done by matching mouse and keyboard players against controller players, I don't think those are relevant, simply because M+K on PC is unrestricted, whereas on consoles with an adapter you are still subject to the same aiming scheme and hardcoded limits as everyone else.
I can understand how someone can watch videos and come to the conclusion that using XIM4 instantly turns you into the John Wick of video games, since those feature experienced M+K users playing against casuals with controllers, but I see plenty of good pad users who are serious about gaming, and aim well, do the same to other pad users.
A proper test, IMO, would be XIM4 users against competitive console gamers. Set that up, record it and upload it.
A Xim is however an obvious advantage in everyway and needs to be noted as one not denied.
I was a competitive controller player for years, have never picked up a Mouse once in my life aside from typing, bought a Xim and in under 5 hours was pulling the same and then higher k/DS than I was previously, getting kills at ranges I never would have attempted with controller among countless other small noticable changes, I have under 70 hours of MnK gameplay and I can confidently say that it feels far superior to my thousands of hours of controller muscle memory, as MnK is far easier to just pick up and use than a controller, it's basic principle. My k/d didn't increase too much per se but oh my kpm was significantly higher, the mouse lets me play more aggressive raising my average kpm from a 2.5 to around 3.5-4 on Battlefield, I played Ow for the first time ever after getting my Xim and Im already ranked globally on leaderboards for it (I would be with controller too but likely not as good)
Its astonishgly scary that most MnK players are still using aim assist too (something I never used as a controller player) so not only are you basically cheating with a Xim you are also letting the game aim with your superior input device lmao.
For me using MnK is a moral gray area, I have millions of MnK-Hackusations on multiple games from when I used a controller lol I actually bought a Xim as a bucket list type thing, I bought it to piss people off because I was accussed of it for so long and holy @#$% I will never pick up a controller again because a MnK is just that, superior, to a controller in everyway.
However an interesting thing to note, in all my experiences with competitive gaming, controller players were actually always the best players, ironically enough with a few MnK players who competed at the same level (tho significantly less ratio wise), I always figured it was due to the fact most people buy MnK and expect to be gods instantly(like me lol), and they were bad PC players who came to console to dominate on console plebs but couldn't compete competitively, but the truth of the matter is while MnK is an advantage, it's about the intelligence of the player themselves and how much of the MnK advantage they can use. I can pickup any FPS game and dominate (and that's not some Dunning-Kruger assumption mind you) with controller or MnK, although MnK is undoubtedly superior, almost the same things can be done with controller, MnK is just far better for muscle memory and will allow bad players to cheese some dumb kills at times, as MnK can do things a controller never will but its still not like PC.
This is why it's a gray area for me, I kinda wish console would get native MnK support on all games cuz it's just better, tho I wish console would also be controller only as it's meant to be, controller just feels so bad compared to MnK. Sony and Microsoft need to just provide and mandate native support honestly, while it is cheating to use a Xim I don't really care, I hardly play games now anyway and I was accussed of it for so long it feels good to have one. Its an advantage but only a minor one for a good player, as when I had controller I still beat MnK players daily, it's just even easier now, I feel like MnK is a larger advantage for worse players especially with aim assist added to that, tho they still wont be able to compete at a high level competitive level unless they already did with a controller.
Overall I just contradicted myself there a multitude of times in my rant,
TL;DR MnK is a major advantage but I can't go back to controller it's just superior, console needs native MnK.