Well, it is also true that english with good grammar and flow is much better to read. Not sure if you can get such flow with basic vocabulary and simple sentences.
- He walked proudly=> He strutted(about)
- He called his boss to tell him he was sick=> He called in sick
I'm not a native but there's so many examples like this where a native proofreader could easily improve my novel, without mentionning the wrond word usages and potential grammar mistakes.