In my world, if you don't know whether an index is likely to be used, it indicates a lack of understanding of what your users are likely to be doing, and/or the nature of the data set. Yes, there are marginal use-cases, but overall, you should understand what most of your users/consumers are doing and optimise accordingly.
Is there anything wrong with this? My thinking is that it could be seen as a premature optimization if an index is created without knowing if it will be used.