The time period is important too. In 'The Cage', Captain Pike seriously considers quitting the Fleet to become a trader of Orion Animal Women. Going from that to piracy, whilst still unlikely, seems at least vaguely plausible.
The idea that, by the TNG era, a Fleet officer could turn to piracy purely for profit is pretty unbelievable. As TheLoneRedshirt
says, there would have to be a very good ulterior motive.