When you ask exploratory open-ended questions in a survey, keep in mind that writing is hard; most respondents expend limited effort in answering these sorts of questions. Without a trained qualitative interviewer, you also will not have the opportunity to rephrase questions, ask follow-up questions, and use other qualitative techniques to determine what respondents really meant by their brief and often unclear written responses.
As an example, consider this exploratory open-ended question about a most recent hotel stay: “What did you like most about your stay at this hotel?” Let’s say a male respondent wrote down, “Food.” What was it about the food? Was it the fact that he could get food at any hour of the day, the quality of the service, the availability of vegetarian items, or what?
What if he said “food” simply because his favorite restaurant in the city is located within walking distance of that hotel? Sure, we could attempt to foresee such an issue and have follow-up quantitative questions to uncover the nature of the answer. But truthfully, many such answers to exploratory open-ended questions leave us hanging. Without a qualitative approach, these issues cannot be explored.
Now consider an exploratory open-ended question that asks why some action was taken. For example, suppose a survey asks, “Why did you decide to purchase this vehicle?” Questions that address why are almost always multifaceted. It might take some time and thought for a person to realize and articulate all the factors involved. In the case of the car, some of the deeper issues might include family considerations, expectations about the future, or self-image, as well as the more obvious factors such as cost, safety, reliability, repair records, and so on. The point is that what most respondents write down represents only a coarse snapshot of the full picture.

Without having an interviewer present to ensure that respondents understand the question, respondents will assume different intentions of the question. When asked, “Why did you decide to purchase this vehicle?” some respondents might write down why they purchased rather than leased the car; others might say why they purchased that car instead of another car they were considering; others might think you are asking why they decided to buy the car now instead of at some other time. Without addressing the question qualitatively, no one knows how respondents will interpret the question.There is nothing wrong with using open-ended questions in surveys. Just recognize that your ability to understand what the respondents meant by their responses is limited. Few respondents write well and take the time to craft an insightful response. Even if some write a thorough response, true understanding might require qualitative exploration.
Of course, the best approach is to conduct qualitative research before designing quantitative research.