I have been studying S&W history for a very long time, and my first firearms book was a copy of McHenry & Roper. Somewhere in the past 65 years I read that the original purpose of the barrel mounted locking bolt and locking bolt lug was to protect the extractor rod from damage if the revolver was used to "buffalo" (pistol-whip) a suspect during an arrest. Reportedly the Colt extractor rod would often be bent when the pistol was used for this purpose! I am not sure if this came from McHenry and Roper, Elmer Keith's writings, Ed McGivern's writings, or whatever, but it was from a printed source many, many, many years before the Internet.
For whatever reason the front locking bolt and lug are not necessary for proper function of the revolver, but the above reason does make sense! And to answer OP directly, NO, the locking lug and locking bolt are not a "gimmick". S&W would not have added this complication and kept it for 123 years now if it didn't serve a purpose!
