At a minimum, the job of an instrumentalist is to play the right notes, at the right time, at approximately the right dynamic levels. A good instrumentalist will also creatively and expressively interpret the music. However, that is not strictly necessary. Generally speaking, an instrumentalist may expect to be paid and applauded for a performance that is simply clean and correct. Essentially the same goes for a vocalist, although a vocalist must also be intelligible. Additionally, there are requirements of etiquette that seem to go without saying: a professional musician should be punctual, appropriately dressed, prepared, polite, etc.
It is reasonable to expect that a trained, experienced professional musician will possess all of these basic skills. That is what you are paying for when you hire a musician.
It does not seem reasonable to expect a professional to perform a task for which he or she is not adequately trained. No one in their right mind would hire a trumpeter to fix a kitchen sink, for instance. There are obvious reasons why that would be a bad idea. Even though a trumpet consists of metal plumbing, a trumpeter is not a plumber. Even though a trumpeter may be an independent contractor, it is in a different field. Even though a trumpeter may possibly own a kitchen sink, that does not mean she or he is knowledgeable about kitchen sinks everywhere.
It also seems unreasonable to expect a trumpeter to give a bassoon lesson. The reasons are basically the same. Even though the trumpeter plays an instrument, that instrument is not a bassoon. It is nothing like a bassoon. A trumpeter is probably more qualified to talk about music generally than fix a kitchen sink. However, that does not mean he or she knows anything about bassoons in particular.
Musicians are sometimes expected to do things for which they are not necessarily thoroughly trained. Privately, I have coined a term for this attitude: I think of it as the "kitchen sink problem". It seems as if some people want musicians to be able to do everything, including fix the kitchen sink. That thought occurs to me whenever someone assumes I can do something for which I am not qualified. For example, although I am a reasonably good accompanist and sight-reader, I have no expertise in transposing music on sight. There is no reason anyone should assume I can do that. It is an unusual skill. Yet I have had several experiences in high-pressure situations (usually auditions) where I was asked, rather casually, if I could transpose the music on sight.
Each singer was surprised by my surprise. Why? What they just asked me to do is not within my expertise. They may as well have asked me to fix a kitchen sink. No, I can't. Sorry.
That is not to say that anyone who wants to learn a variety of skills should not do so. There is nothing preventing a conductor from also being a fine horn player. There is nothing preventing a musicologist from also being an excellent composer. However, it would obviously be unfair to expect horn skills of a conductor or composition skills of a musicologist.
Here is another anecdote. Recently, I introduced myself to someone as a composer and pianist. The person smiled, introduced himself, and asked me if I also sang. No. Why not? he asked.
He may as well have asked why I don't crochet. I do not sing in professional circumstances because I am not a professional singer. (The shower does not count.) I am not a professional singer because I do not have the prerequisite vocal performance training or expertise. To say that I could or should sing simply because I play the piano and compose seems to devalue all three pursuits. It seems to imply that those things are easy, or so closely connected that once one of those skills is mastered all of them are mastered.
It is true that there are successful self-taught and untrained vocalists. There is no reason to assume I would be one of them. It is hard to sing well. It is time consuming. If I were to pursue vocal performance seriously, I might have to neglect some of my other musical projects. No, thanks.
Perhaps, deep down, we are eager to assume that artists will exhibit talent or ability equally in all creative fields. Maybe we want to see polymaths everywhere we go. The success of the visual artwork of Ringo Starr, for instance, seems to hinge upon the belief that he should be celebrated as an artist because he is celebrated as a musician. That is just a speculation, though. I do not know where this assumption comes from. I just wish people would cut it out.