In colder months, I carry a J-frame in the weak-hand outside pocket of my coat. It functions both as BUG and as the immediately available firearm, if my coat is closed. If I had to shoot it from the pocket, I would, but in addition to suffering loss of accuracy, there are at least two other factors that would concern me.
First, in a physical confrontation, I'd rather have both hands and arms free. A hand in a pocket effectively ties up my hand and my arm, and depending on the scenario, fending or striking might be more important than shooting.
Second, try moving around rapidly, turning, and changing levels with one hand in a pocket. It has a major impact on balance. Depending on the scenario, moving freely may be more important at any given instant than shooting.
So, is shooting from the pocket worth practicing? Sure, if you can afford the training jackets... OTOH, actually shooting from the pocket in a defensive scenario would be a last-ditch method, IMO.