MBTI is a tool for describing someone's basic cognitive preferences and to outline a set of characteristics which should in general be accurate. It is as accurate as saying that a subject is dependable when that subjects own actions have shown them to be so.
If it is used to prescribe someone as having to act in a certain manner or used to limit a person in any way then it is not being used correctly. Ergo it does not follow the theory to ascribe any value of healthy or unhealthy to any function or orientation there of. You may say that someone is stressed when they are showing negative signs of a type as described by INTJMom but it is not valid to say that Ti, for example, is unhealthy for any type. The only means with which you can use the MBTI to define healthy or unhealthy is by using it as it is designed to be used which is a framework for communication.
For example, my father is an ENTJ. He is trained and somewhat want to listen to people attentively and compassionately, this is not a quality which is attributed to an ENTJ. It is, however, neither healthy nor unhealthy. But to put him in the position where it is necessary to continue to do those things as a means of survival then it would be an unhealthy position for him and one form of helping him with that problem would be to try and get him to use his more dominant preferences for a while as a break.
This kind of healthy/unhealthy situation is also exampled by my former employment situation where I worked in a hostile environment with a bunch of competitive ESTJs and ESTPs. My adaptation to that environment and the coping mechanisms I used were not unhealthy but when I got home I'd spend 10-15 minutes with a book relaxing in order to release the tension being in that situation caused. Were I to arrive home and continue to bring that hostile environment with me (as I did on occasion) I'd display characteristics which would be unhealthy for me as they were no longer in context and were a result of me not being able to cope on that occasion. Some of these signs could be assigned MBTI attributes by those with a want to do so but I would still be an INTP no matter what I was doing. Assigning MBTI attributes to things which should not have such things is not a fault of the system but a fault of the person employing the system.
How am I defining unhealthy, that would be a state where I or someone is showing characteristics which do not serve to further themselves in terms of survivability (instincts), popularity (social) or goals. I'd guess if you wanted a good idea of where those kinds of measurements came from then I'd suggest looking into Maslow's hierarchy of needs, though I have my reservations about the whole "stage 1 needs to be done before stage 2 can happen", personally I think they under use the idea of undermining.