People don't make character choices on MUSHes because of optimal mathematics. They make character choices because of perceived coolness. Having a lot of dice to throw at a certain task is a factor here, but it's not the only one. Other factors include story elements and prominence in the setting. A sometimes-problem* with many settings, including The Witcher, Dragon Age, Star Wars without the Extended Universe, Shannara, the Dresden Files, the X-Men, the World of Darkness, Exalted, etc. is that you have most of the canon story wrapped around a small group of elevated individuals. In Exalted, there are exactly 200 Solar Exaltations in Creation, and not all of them are incarnated at any point in time. So, because the well-meaning game runner wants diversity and a rich game world, they allow and maybe even encourage people to play "mechanically disadvantaged" characters, who can't possibly be on the same level as the characters who take center stage in canon. A number of people (like me) take up that offer, and you end up with a small cadre of "normal folks" hanging out with a bunch of superheroes (which is actually canon-appropriate for some of these settings). Maybe the normal folks have access to their own resources, but you know, because they're lower in number and don't have as much canon plot support, they might not get quite as much attention from staff as the supers. They certainly can't participate in all the plots, whereas the supers can easily participate in "normal folks" plots (and sometimes can make those plots feel anticlimactic). Over time, many of the "normal folks" players lose interest, focusing on their supers alts. The ones who are committed have to do more and more stuff with their supers-playing friends, but they're still locked out of a lot, either because of canon or because they can't keep up mechanically.
So no, you can't blithely say, "Oh, people can choose to play mechanically disadvantaged characters," because that doesn't work. Some people will have some fun with it, and then that part of the game will wither and die because everybody's playing Witchers and Sorceresses and occasionally you'll have someone playing Vernon Roche and min-max their stats to a point where they can compete. If you want a setting where people with magic and people without magic can play together as equals, the magic has to have costs and you have to make a concerted effort to have prominent setting elements that have nothing to do with magic. Then a greater number of people will participate with the "normal folks" side and it will retain more momentum and interest.
* "Sometimes-problem": it's a feature, and it's not always a problem, but sometimes it is. The problematic element is usually amplified with the changes necessary to translate an existing fictional world or a tabletop system to a MUSH.