They are both stand alone episodes. The Menagerie could have been it's own, returning to The Cage drama, to complete the story of Captain Pike, which was developed for the show, him being gravely injured; more importantly, the duel episode is a very key event in the story line relationship between Spock & Captain Kirk. Spock shows uncanny loyalty to his first Captain (assumed), Captain Pike, by risking his career and and not just that, but Spock shows compassion, not at all a Vulcan trait. With the episode being in the first season, early in the mission with Captain Kirk, that character development for the two leads in TOS was very valuable.
Relationships grow or fracture when under stress, and this episode, within the story line, would have revealed to Kirk that Spock's devotion to him as his Captain was promisingly destined to be rooted in a form of deep friendship, which we know was rooted partly in the human genes of his character. That is the character of Spock, an emotion-controlled Vulcan, with surface accents of emotion, many which run deep, brought forth by his human side.
Despite being brought on by budgetary causes, I think The Menagerie was a story line that set the stage for the character of Spock more than any other episode. I see it as the foundation for why the character of Spock became much loved; among other things, he was willing to disgrace himself for his friend, his captain. In see it as a book-end of his loyalty as a first officer, in this case to Captain Pike, then to Capt. Kirk. The other book end is when he dies in ST: Wrath of Kahn, saying to Kirk, " I never took the Kobayashi Maru test, until now; what do you think of my solution (Kirk always facilitating a safe way out of trouble)?" Spock then says, " I have been, and always shall be, your friend; live long and prosper. "
