And actually, I wanted to add some more to this but was unable to locate exact reference earlier. Thanks to the other thread from Dr. Bhate and Dr. Kannan about usage of the word ashtadhyayi in bhashyam - I was able to locate it. Here it goes, quoting from पृषोदरादीनि यथोपदिष्टम् 6.3.109. Very beautiful (somewhat hilarious as well), this precisely answers your question (Viz. why study grammar instead of just asking others who already know it)
यदि तर्हि शिष्टाः शब्देषु प्रमाणं, किम् अष्टाध्याय्या क्रियते ? शिष्टपरिज्ञानार्था अष्टाध्यायी? कथं पुनः अष्टाध्याय्या शिष्टाः शक्या विज्ञातुम्? अष्टाध्यायीम् अधीयमानः अन्यं पश्यति अनधीयानम्, ये अत्र विहिताः शब्दाः तां प्रयुञ्जानम् । स पश्यति - नूनमस्य दैवानुग्रहः स्वभावो वा, योऽयं न अष्टाध्यायीमधीते, ये च अस्यां विहिताः शब्दाः तां च प्रयुङ्क्ते ! नूनम् अयम् अन्यानपि जानाति इति । एवमेषा शिष्टज्ञानार्था अष्टाध्यायी ॥
When a student of ashtadhyayi sees someone around who is easily and confidently using the words (but never appears to study ashtadhyayi), then there is a high chance that he is a scholar and he knows a little more as well (even the words that are not in the ashtadhyayi). This is how one can identify the real "shishta" and approach them.
Bottom line, the "study of ashtadhyayi" and "consulting the experts in the field" should typically go hand-in-hand so that the error from the one is balanced by another!
इत्यलम्
नीलेशः