Native Speakers V.S. Linguistic Specialists
It is great to be surrounded by native speakers while learning a language, especially for children, because children's brains store and access information in a very different way from the grown ups. Children's brains are able to quickly memorize, process, and analyze large amount of data in a very short time, and that's the case for most of native speakers to learn their first language. For me and many adults, memorization takes more than repetitions. In other words, for me and many grown ups, remembering something takes extra emphasis. In the case of learning how to pronounce things properly, remembering diction rules is less work than remembering the pronunciation of every single word. Once you know the rules, you don't have to memorize the pronunciation individually anymore! And because of remembering the rules(and learning the meaning of words), it helps singers/actors tremendously in memorization, because our brains built extra bridges to connect and strengthen the access to our memory.
A native speaker can easily demonstrate the proper pronunciation. Depending on how the native speaker learned, he or she might not be aware of the rules of pronunciation. For instance, two of my Russian friends told me that Russian is the kind of language that you could pronounce the way the words are written. When I asked them why the three "o"s in the word "хорошo"(which means good) were pronounced differently, they were stunned. They took diction rules for granted because their brains did all the grammar work for them unknowingly when they were young. After I described the rule, "the stressed o is pronounced as open o, and o before the stressed syllable is pronounced as [a], and an unstressed o not before a stressed syllable is pronounced as a schwa", I could see their brains going through their database trying to verify the information. After a while, they both said, "You are right! I never noticed that before!" (You could figure out which one is which by the clues I've given here :p) The more ironic part about the story was that I actually learned the rule from the Russian textbook one of these friends gave me. This once again proved that native speakers could take pronunciation rules for granted. At least one of my friends skipped the part describing pronunciation rules in the book he used in college.
A linguistic specialist has his/her specialty which could be the amount of words he/she knows in certain language(s), how the languages were formed/created, how the characters were developed, how to pronounce,... In order to find someone who can teach us good diction, we need the specialists who are very good at explaining diction rules, good at demonstration, good at catching subtleties in speeches, and good at teaching us how to phonate in addition to pure repetitions.
Nowadays, it's pretty easy for us to look up meaning of words and IPA (International Phonetic Alphabet, not the beer IPA) online. However, readers who see IPA need to understand what the symbols represent as sounds, and it may take way too much time if we have to look up every word for the meaning and the pronunciation. And this is when you really could use a diction coach like Ü. After learning the rules, you'll know what to really look out for to save your time, and where to search for the answers so then you can save time from your private lesson or coaching for more artistic elaboration.