While I would debate that incorrect data is worst than no data, that's not relevant here. I'm speaking purely from an efficiency point of view. I know that most players actually come from the US (if not the majority, it's the largest minority) but JP by itself is a good representation of a majority, and as an added plus, its native language. Talking specifically about the US (I guess you can use NA as a whole), since we have freedom of speech and we are a country of immigrants, our opinions will likely differ from each other and we will end up with contradictory feedback. Since Japan is has more of the hive mind (not saying this is bad) mentality due to lack of the former, it's easy to take the opinions of a smaller group here and be confident that it will represent the opinions of a larger group. So for efficiency sake, they would (and rightfully should since it's more efficient in every way) just look at a few sparse topics in the JP forums and compile feedback based on this. Rather than try to read hundreds of threads with differing opinions and flaming (they tend to keep relevant stuff in one topic, rather than making 10 topics and also stay on topic in the posts) then try to make a decision as to which side they should take, then go to the translator and hope the rather colorful English actually convey what we want (we also tend to use sarcasm a lot in English, which doesn't help...).
The obvious solution is to:
1. Not make 10 topics on the same subject and just keep posting on the same topics
2. Keep irrelevant stuff (e.g. ad hominem) to a minimum and post what you want them to actually take back to the dev team
3. Be serious when posting. All it takes is for them to take one sarcastic comment seriously, and we get the opposite of what we want because that's exactly what we said. This also means eliminating the troll topics and troll posts.