When learning a foreign language, most students/learners prefer or even require that they learn from a native speaker of that language. The common perception is that only a native speaker truly understands the target language, and, while that may appear to be true at first glance, this bias deserves further examination.
It is a well-established fact that native speakers commit grammatical errors on a regular basis in their – especially – spoken language (between 10 and 15% error rate on average). This is very evident if you record a native speaker in conversation and then attempt to transcribe it – you will encounter run-on sentences, many starts and stops, self-corrections, filler words (e.g. umm, you know, like, etc.), and errors in word choice. Despite these errors and distractions, the native speaker is still easily understood and can communicate effectively with others in their native tongue. So, why does someone who also speaks that language fluently (near-native speaker) appear to be less reliable than a native speaker (NS)?
First, let’s define what qualifies as a near-native speaker in a given language. This category, near-native speaker (NNS), is one that goes beyond most level descriptors and is nearly on par with a native speaker. In fact, NNS is the highest designation attributable in terms of proficiency. Minor differences between the two types center around intuition about the language and its underlying grammar. In a sense, then, the performance (actual words spoken) may not differ much between both NS and NNS, but their competence (base knowledge and ability) may vary quite a bit. That is to say that a NS and NNS can both produce language that grammatically accurate, but the NNS generally will not have the same feel for what “sounds right” to him or her. Keep in mind that, although we sometimes say certain things, it does not mean that they fit a prescriptive grammar for the language. Since languages are constantly changing over time, NS may have a small advantage over what is acceptable as an utterance outside of the standard language, particularly in the spoken word. This is, however, something that NNSs can also know, especially if they stay in contact with the language.
Second, although a NNS may not have the same intuition about the target language as a NS, what explanation does one get from a NS when questioned about a particular sentence or turn-of-phrase? You have probably experienced this at some point yourself when asked something like, “Does this sound right to you?” or “Can I say ____?”. When you explain your intuitive understanding of what “sounds right,” do you explain it grammatically, or do you just offer what you “know” is correct? Are you wrong despite an inability to explain the grammatical reason behind your intuition? No. By the same token, if a NNS produces a grammatically correct sentence for the person asking the question, is it any less valid than what the NS says? I would suggest that there would be only a minimal difference depending on the context. For example, if a near-native speaker always speaks precisely with attention to producing grammatically correct sentences, they might be perceived as “stiff,” “robotic” or “unapproachable,” but so would a native speaker in the same situation.
Some may claim that a foreign accent interferes with comprehension and reinforces non-native-sounding pronunciation. One could argue that, given the usual resources and materials used in language instruction (e.g. audio recordings, videos), not only do these correct for a slight difference in sound (near-native speakers do not exhibit noticeable foreign accents), but they also potentially offer insight into different dialects of the target language, depending on the variety of materials selected. Further, at the most basic levels of language instruction, learners do not have a fully trained ear for the sounds of the target language, so minor differences in inflection are not usually noticed.
 See also https://en.wikipedia.org/wiki/Near-native_speaker.
 A prescriptive
grammar is what is taught as the “standard” of what is and is not grammatically
correct in a language – think of it as formal language, as opposed to the
everyday language used in conversation.
 I put “know” in quotes, because it may not be grammatically correct, though dialectologists and linguists maintain that strict adherence to the standard is not a condition for comprehensible language.