All extant belief systems are equally fit, by sole virtue of their existence, however their existence and present state is not accidental, and lends a great deal towards their fitness. Consider, for example, a system that exhibits strong polarization (let's use United States' politics → as our example). This might seem like a bad thing, something that that system's caretaker, were it to have one, might want to prevent or fix. To jump to this conclusion is to ignore the usefulness that polarization brings to a system.
Specifically, polarization allows a system to tack or make deliberate course adjustments. Each polarized head represents a moderate level of consensus which, combined with the fact that there are multiple of them, end up functioning as choices or alternatives. In our US culture example, voters get to choose from between two viable options, a choice that isn't available in systems dominated by a single normal/party. As a result of this choice, not only does the belief system as a whole have the power to consider alternative realities, it crucially gets to select whichever one appears to be more fit.
Furthermore, if the heads in a polarized system are competing against each other (say political parties seeking votes), then whichever belief system is less favored (say out-of-power) has a strong incentive to deliberately re-orient itself in a direction that it feels will be more popular or normal.
As power passes back-and-forth between these competing belief systems, the system as a whole ends up tacking, first turning one way, then another, as it moves into the future. This tacking behavior, because it alternates direction, gives the system as a whole greater power to change its direction, seeking out profitable beliefs and restructuring in the face of challenging situations.
Note that there are two degenerate ends of this tacking strategy. The first is the case where all polarized options closely resemble each other (vector-wise, they can be said to have a small angle between them). In this case the system starts acting as though it were normalized, merely going whichever way leadership wants it to go. The second is the case where all polarized options become anti-thetical to each other (vector-wise, they can be said to have a large angle between them, something that approaches 180 degrees). In this case the system's tacking behaviour becomes erratic, as each change sharply contradicts the previous trajectory, and as forward progress is stalled in favor of repeatedly undoing previous changes and doing changes that were previously undone.
Needless to say, in our US culture example, the bi-polar system is approaching this second degenerate condition, with the Democrats and Republicans playing a game of who-gives-less-ground with each other.