This Inside Higher Ed article about syllabi at CUNY specifying "don't call me professor" is making the rounds of my FB circles, even though it's nearly a year old (perhaps because a conversation about adjuncts is revving up at my institution in anticipation of ).
The article sets the practice of telling students to avoid the p-word within the context of adjunct activism at CUNY and beyond. The article describes some adjuncts who raise the "what's my title" issue as a way of tipping students off to the stratified and often exploitative nature of academic labor.
Even among adjuncts who have a comfortable berth, professional titles are an uncomfortable issue. Try it: ask adjunct faculty how they wish to be addressed by students, and chances are they'll still be talking five minutes later, not in an angry way, just in a "yeah, I don't know how to solve this problem" way. There are many options, but none of them fit comfortably. "Professor" is usually inaccurate (though my institution has a few "visiting assistant professors" and "research associate professors" who can use the title in good faith. The titles that many adjunct faculty do in fact have don't customarily function for purposes of addressing people, so no one even tries to use them that way, (e.g., "Lecturer X"). "Doctor" is fine if one has a Ph.D. and the institution is one where the title is commonly used, but in institutions where only MD's go by "Dr.," the person insisting risks sounding pretentious and out-of-step. "Mr." or "Ms." conveys to many students that the instructor is a graduate student with limited mastery of the subject or control over the terms of the course--even when that's palpably not the case. First names may be comfortable, particularly in small discussion-centered classes, but some students feel awkward using them (particularly to older instructors) and reach for a title anyway.
As adjunct problems go, this one is minor (far better to be worrying about titles than health benefits, salary, job security, office space, or any of the many other things that can go badly wrong off the tenure track). It's no more or less than a reminder, every semester as one confronts a new set of students, of the mismatch between the work of teaching and the institutional norms that validate that work.