Some questions are poorly worded or ambiguous and so a yes for this reason and a no for this reason is a valid answer. Additionally, some questions just don't have a good answer, we just have to consider the question "Is it okay to lie". The answer is not a yes or a no as there are always examples where you would disagree with your answer.
There are sometimes when you want to agree and disagree with a question, but you can't. There have been a lot of opinions I couldn't post to because I was in between on the subject and I couldn't say yes or no specifically. An "undecided" column, if you will, would actually in my opinion be helpful.
It would make answers easier, but It should be "neither yes or no" rather than undecided." An undecided person has no reason to be in a debate, but arguing that there is reason for yes or no is viable sometimes. For example: in the debate over whether people with ADHD should take meds I wanted to say that although stimulants tend to be over prescribed it should be up to the family and person afflitiated, but I had to make a binary yes or no choice.
Being undecided is not a position that is meritorious let alone invaluable and worthy of integration into a system. "I don't know" or "Maybe" or "Well, I can see both sides" is by nature not particularly well reasoned or strongly asserted therefore it is not something worth pursuing to incorporate.