I believe the spectacle Mic/Intercom switch is spring loaded from Mic to Off and not spring loaded on the intercom position. So that switch may be left in the intercom position to activate the microphone, the headset always being live if selected on the station box.
The 737, being sloww, has a nice quiet flight deck, so intercom on continually is not that irritating. Faster jets with more noise tend to preclude use of continual intercom 'on'.