75% of the best fighters train with weights, sure. I'd love to know how many seriously trained weights while they were white and blue belts in BJJ, or whatever art they have the skills in. I bet GSP was doing sweet FA weight lifting while he was honing his karate, and most other of the best (other than Brock). I know you can see a picture of Fedor when he was young lifting weights with his brother, but I suspect most champions have had a weightlifting stint when they were young, you could have hardly survived the living through the 1980s and Arnold/Stallone movies without getting the urge to at least try it for a while. This doesn't mean they kept it up while they were developing their skills, and that is where the TS is in his MMA journey.
Unless you have a large inheritance, really nice parents, are a teenager or are independently wealthy by age 25 or so (i.e. can afford to train full time), I don't know how you can juggle weights and BJJ when you are starting out. Doing the weights just saps too much time and attention away from learning skill, IMO.
It's not as if I haven't lifted weights before - did about 2 years in college (including squats and deadlifts), and later, trained at a top powerlifting gym for 6 months or so, and also lifted weights for other sports in between. Doing the powerlifting I got very reasonable results for my size and absolute lack of juicing. Maybe I could have eked out an extra 10-20% with an extra 3 years of training. Maybe. To what end? That's 4 times a week commitment that will certainly edge out your grappling. And guess what, after that 6 months of powerlifting I would have been way stronger than pretty much anyone I have ever grappled who was my size. And I would have also gassed in approximately 30 seconds.
Funnily enough, I did the powerlifting in order to be able to dominate one guy I grappled with. (The guy was naturally bigger, and a manual laborer, so pretty much a beast). The thing was, I lost interest in the MMA because the powerlifting was taking over my time anyway. And I don't even know if the guy I was concerned with trains BJJ any more.
When I eventually got back into BJJ, I was where I had left off (pretty much), and was weak again because I had stopped the PL. But I guess I had learned some good lessons:
1. You can get really, really strong in only 6 months of lifting weights. (Far) stronger than you need to be in order to do well in grappling comps.
2. Strength will only make a difference in a bout between two similarly skilled grapplers, assuming identical weight, and all else being equal. More than a belt, my money is on the skilled grappler.*
3. You most likely have a limited number of hours per week to devote to grappling, weightlifting, whatever.
4. It takes forever to be really good at BJJ. Black belt takes anywhere from 3-10 years, and the minimum was BJ Penn, and AFAIK he trained pretty much full time. Unless you are independently wealthy and a natural, it will take you longer than BJ Penn. And black belt is not even a guarantee at MMA success. There is a huge gap between Saulo, Roger Gracie, Marcelo Garcia, BJ Penn, and the average black belt. But this skill level is probably where you want to be, unless you are a talented striker or takedown artist.
5. All hours of BJJ practice aren't created equal. 10 hours of BJJ practice in one week will give superior results compared with 10 hours stretched over 2 weeks which will in turn have superior results over 10 hours stretched over a month. This is because your brain can build on what you learn, you will mull over it more, and you will build on the previous stuff before you can forget it.
6. If your goal is to win competitions that matter, you are far better off training strictly BJJ until say, somewhere between purple and black belt (depending on whether your goals are MMA or BJJ. For BJJ success or a BJJ oriented MMA game, go longer without weight training).
7. Another good side effect of not doing the weight training is that your inferior strength will force you to use good technique, because you have little muscle with which to "muscle". This gets you better, faster.
8. Take home lesson: forget the weights until you are at a level where it matters, and until then beat people where possible based on your superior technique. When your technique kicks ass, spend 6 months in the gym and then go tie people up into pretzels.
*In fact, I have actually gone up against someone in a comp who was training powerlifting at the same time as he was doing BJJ. The guy was an intimidating beast. I was doing no weight lifting at the time, only grappling a lot in class (maybe 4 times a week). We had both done BJJ for approximately the same length of time, and he was a good, technical grappler. I secured the takedown, got side control, mount, then choked him out. (BTW this is not to say that I am better, but I was on the day.) Afterwards someone told me that he had said that I don't look strong, but that I was deceptively so.
I'm not sure what I'd put that down to. I remember around that time I tried doing 135lbs for bench, and I found it tough to get more than a few reps out (shoulder was hurting), but it also felt heavy, too. Pretty pathetic. My personal best 1RM raw bench was 275lbs. So maybe I was about 60% of my former strength, at least in bench. (I'm not built for bench btw, I'm built for deadlifts. But totals are indicative at least.) I suspect that the 3 years or so weight lifting experience at least teaches a person how to recruit muscle fibers when necessary, and you retain some of that. But more than that, I know that once I have side control I am hard to budge, especially if I am lankier than my opponent. And it doesn't take much muscle to make sure that all your weight is on your opponent's chest, through your chest, while you push off your toes.
What does that one competition result prove? Nothing, but it is an indication that the stronger athlete (especially in a weights sense) will not necessarily prevail. The little judo and wrestling experience I had to date was more than his (my BJJ was probably similar). I also had a game plan - not to cede the takedown, and if I could land in side control, I could most likely maintain it (or my opponent would gas trying to escape), and I could probably get a sub. My speed enabled me to help get the sub, but it was the skill that allowed me to be in the position to get that sub in the first place.
Another thing is that skill is effective strength. I think we have all seen purple belts and above who appeared really strong, and we've told them so. And then they reply that "are you sure it's strength? I think if you notice what I'm doing, I'm being clever with the leverage...", and it's true. If you put all your weight on the very end of the lever, normal to the direction you want to torque the lever, and in addition, try and get that lever in a position where the opponent has little leverage on that lever, you will appear far stronger than you are, but it has nothing to do with strength, only skill. It takes a few years to go from being able to understand that concept to actually being able to consciously apply it when rolling.
YMMV.
Edit: You know what I'm surprised at, is that there aren't more fighters who make up bs about how they train in order to derail promising juniors. Someone like Anderson Silva could state that he has a 400lbs bench when he would be lucky to do 200, get M&F to print up some BS training routine he does that would be quoted forever more, and all the noobs would be doing it thinking it was the ticket to greatness, while providing a big impediment to them ever amounting to anything. And even some of his opponents would be spending time in the weight room when they should really be working on how to not run after him with their hands down and their jaw out in space.
