Certain fields have barriers for entry. To do what I do, for example, I have to be able to speak and write Spanish at a high level. I see this as a barrier because it is simply a given. Since it's something I share with everyone else in my field, it doesn't set me off from the crowd. (Nevertheless, there are Hispanists who haven't mastered certain details of phonology yet; so maybe it's not a barrier to entry. In the UK Hispanists did not [traditionally] speak Spanish well though that has changed considerably in the past generation) I wish I had even more mastery of the language than I do, but it hasn't held me back.
The high-wire artist has to be able to stay on the wire. Whatever other tricks she can perform in theory or on the ground don't mean anything without that one skill.
The barrier is not a trivial one. Suppose I wanted to be a Sinologist but didn't want to memorize endless characters, or be a musicologist without knowledge of harmony. I might even have interesting ideas about the T'ang dynasty or about music, but I wouldn't be taken seriously. That's the medium in which experts in the field have to swim. (Water polo players have to learn to swim first; hockey players, to skate.)
It's a complex question, because the barrier to entry can get confused with competence in the field. A "heritage speaker" might face discrimination in a Spanish Department. Or a minor flaw in phonology might disqualify a candidate who is otherwise superior.
It's difficult to enforce a high level of writing among Spanish majors. Since we don't view undergraduates as (potential) colleagues, we don't accord them the respect of demanding a minimal standard of writing in the Spanish language. If we demanded that level, then we would be effectively creating a barrier to entry into the major itself, or spending all our time teaching syntax and never really getting to the content of the literature and culture courses.
The English major can do a backflip on the ground; the Spanish major has to do it on the high-wire. We are constantly having to say: "that would have been a good flip, had you not fallen off the wire."