Do you honestly think you can judge an English script unless you are fluent in Japanese? By that logic every editor should be fluent in Japanese.
Say someone who is a decent judge of writing but knows no Japanese is asked to evaluate two scripts. One is a well-written dialogue that makes sense based on what happens on screen, but is otherwise completely made up, and one is a somewhat stiff but accurate translation of the Japanese (which in itself may be less than perfect). Which do you think they will decide is better?
A fansub is first and foremost a translation, and while there is a lot of leeway to get creative and embellish, the most important thing is that it gets across the right ideas. This may involve lines that don't make sense immediately or less than perfectly-written plot elements that some might be tempted to whitewash over with alternate versions of the story that make sense to them. However, a translation that does that fails as a translation. Without the ability to evaluate the translation, or at least understand why the script is written the way it is, you cannot evaluate the fansub. I've seen many criticisms made of scripts that were flat-out wrong because the person writing them didn't understand why the lines were written that way.
If instead of accuracy you simply want the script that will sound best to you, by all means take your advice from people who don't know Japanese. This is especially problematic with weekly shows, however, as rewritten or mistranslated plot elements can easily come back to bite you later. But yes, you cannot tell translation from scriptwriting unless you know Japanese, and a translation that fails at translating kind of defeats the purpose.
Editors don't necessarily have to know Japanese, but they also don't work alone. Ideally they should be asking their translator for clarification about unclear lines and the translator should also be making sure they don't misinterpret anything or go too far in their edits. When editing a simulcast script, unless you are very conservative in your approach, it's very risky to not have a TLC or at least someone you can bounce confusing lines off, because you're fairly likely to insert errors without some kind of oversight.
A fansub is first and foremost a translation, and while there is a lot of leeway to get creative and embellish, the most important thing is that it gets across the right ideas. This may involve lines that don't make sense immediately or less than perfectly-written plot elements that some might be tempted to whitewash over with alternate versions of the story that make sense to them. However, a translation that does that fails as a translation. Without the ability to evaluate the translation, or at least understand why the script is written the way it is, you cannot evaluate the fansub. I've seen many criticisms made of scripts that were flat-out wrong because the person writing them didn't understand why the lines were written that way.
If instead of accuracy you simply want the script that will sound best to you, by all means take your advice from people who don't know Japanese. This is especially problematic with weekly shows, however, as rewritten or mistranslated plot elements can easily come back to bite you later. But yes, you cannot tell translation from scriptwriting unless you know Japanese, and a translation that fails at translating kind of defeats the purpose.
Editors don't necessarily have to know Japanese, but they also don't work alone. Ideally they should be asking their translator for clarification about unclear lines and the translator should also be making sure they don't misinterpret anything or go too far in their edits. When editing a simulcast script, unless you are very conservative in your approach, it's very risky to not have a TLC or at least someone you can bounce confusing lines off, because you're fairly likely to insert errors without some kind of oversight.