While something like avoiding allergic acne may not motivate people to stick to a special diet, other conditions, like lactose intolerance and celiac disease, can cause extreme problems and it's certainly worth sticking to a die to get relief. For some conditions, such as diabetes, not following a special diet can even be fatal.
When people first learn they must avoid certain foods that often sounds extremely difficult, but over time special diets become much easier to live with. Reading ingredient lists on labels becomes a habit and substitute foods that once seemed strange become normal.
Once people become used to their diets the biggest problem is likely to be other people insistently offering forbidden foods. If someone refuses a food it may seem a little rude, but may be less so than if they were to give a graphic description of what would happen if they were to eat it. And it's not nearly as impolite as trying to force a person to eat something they shouldn't.