Does the phrase "Who are you...," when used as an argument against someone's suggestion, hold any logical merit?
Must one necessarily hold a position of authority (i.e., paid teacher, paid philosopher, etc.) to afford logical advice on any given subject?
If not, how can this phrase be rebutted to a certain satisfaction of the one who spoke it (assuming the speaker of the phrase is a logical listener)?
No, you don't. On the other hand, it is as well to know what someone's qualifications are for expressing a view, especially if we are relying on that view for something important. Bona fides might not be enough, and we would have the same problem of wanting to know how to establish that also.