Completely legitimate to me. If you are Computer Science professional there are good chances that you can either argument informally why the problem seems to be hard, or (even better) provide a sketch of reduction from a known NP-hard problem.
Many real world problems eventually turn out to be NP-hard, and stackoverflow also has now and then questions about the complexity of a problem which turns out to be a difficult one (NP-hard, for instance). It is an important part of a CS professionals toolbox to be able to recognize and to argue for problems which are known to be difficult to solve.
