When I ask students what field they are interested in, I usually do it to get to know the students more, to know what they like and dislike, and also to try to tailor their rotations to stuff that they might need to know that is related to their fields (ie, if interested in ortho surgery, then discussing how medicine docs risk stratify patients, or treat common complaints that everyone will field such as chest pain, shortness of breath, etc)
Every field is interesting and every field is important, so I try not to demean anyone. I usually say "good for you" and "that's a tough field, I know I can't make it in that field" ... and "we need more people like you to go into that field" ... which is true. We need good dermatologist, good histopathologist, good interventional radiologist or even pediatric neuroradiologist (who can tell the difference between a normal MRI of a premie brain vs diffuse atrophy with loss of gray white matter differentiation concerning for possible stroke, cerebral edema, or encephalopathy, clinical correlation highly recommended)
Making fun of other fields or insulting other fields (in front of the students who are interested in those fields) is unprofessional and makes you and your profession look bad and immature.
The student is rotating on my service (whether by choice or not) - my job is to make the most out of the student's time and have him/her learn the basics of my field ... even if the student only remembers 1 or 2 aspect from the rotation (and carries with him/her throughout his training), my job was successful. If I convince the student to go into my field, even better