John Reese is a highly trained ex-CIA operative working with Harold, who is a billionaire. He can buy all kinds of weapons, as we can see in the movie.
However, John never uses a silencer for any of his pistols. He usually shoots in places with a high number of people who can hear the shots. Wouldn't it make more sense to use a silencer?
All of his enemies (aka professional assassins) use them. So why doesn't he?