Probably "None of the above" or even "All of the above".
Like anything else, clarifier is a tool to accomplish something with your water. Most of the time you can get that same shimmer from proper chemistry. Routine clarifier use can cause filter problems, so in that case it isn't necessary. People are often tempted to use clarifier as a Band-Aid, and in that case it is bad because it can mask the symptoms of other problems and is a waste of money. In the case where the chemistry is good, the filter is working and there are no other issues, occasionally a pool owner may need to hurry along the process of clearing the water for a party or event and a rare dose of clarifier in that instance is okay.