The advantage of having learned and used each of the different stances up until now is, you are a lot more readily adaptable to things than someone who only knows one and that's all they shoot.
They can all have their uses, even if it might be only a one trick pony sort of thing (the Quell system certainly is.
), but if you dont know, understand and have experience with them and why, you tend to miss out. And if you have to do things youre not used to, you may just maybe try and make things up as you go, and that really isnt the way to go.
Where you really see that using just one type of stance falls apart, is when you start to move and shoot. They all work for the most part when shooting statically, but once you add movement, and different directions, speeds, etc, things change pretty rapidly, and one may have a definite advantage over another.
Moving straight forward, Isosceles works pretty well as you move, but break off to the right and try and shoot a target to the left, especially quartering away, and it doesnt work, and the Weaver picks it right up, and naturally so. Go the other way, and in the same sort of situation, moving off left and dealing with a target off to the right and quartering away, and your shooting one handed, and again, naturally so.
You need to figure out what works naturally best for you, without having to think about doing it as you shoot, and let your brain just morph through things as you go and as it sees fit. The only way to do that, is to actually spend some time and ammo doing that in practice.