Personally, I think such questions may be a challenge to answer, but I think they are very useful if we can answer them. As long as the metrics are measurable, then data gathering such as this can be quite valuable for seeing what has worked for other communities. We need to be careful with such questions to make sure they remain clear and well defined, but I think when they are well defined they should have a place in the site.
It is true that the exact definition of a category is subjective, but I believe it falls under the good subjective side of Good Subjective, Bad Subjective. By nature, a lot of what we are going to cover on this site will have multiple ways it could be broken down. What is important is not necessarily that there are multiple ways it could be broken down, but rather that whatever way is selected, the correctness of the answer can still be judged.
There are multiple possible ways you could define a category such that the most common is slightly different, but every answer would add value as it would reveal more towards the question and can be clearly supported.