Hypnotism will work for non-subscribers. However, it will not work every time (subbed or not, makes no difference). It seems to be a random, chance-based thing.
When you ask a suspect if they know of anyone with a motive, you have a chance to hypnotise them (it's automatic, you don't have to actually DO anything), thus getting one more name from them than they would have given. However, there also is a chance that it will not work. And there is no guarantee that the extra name is not someone you have already uncovered.