This is probably the closest we'll get to see how he might have conducted on the scoring stage. There is also that Documentary where he conducts with Carreras and Kiri.
I'll bet he didn't conduct any of the 732 performances of the original show at the Winter Garden (and Broadway) Theatre in the 1957-59 period either. Max Goberman, perhaps?
....or ON THE TOWN, from 1944 to 1946.
In general, Hollywood didn't encourage Broadway people coming to Hollywood, and Broadway people didn't encourage Hollywood people coming to Broadway. They are very different worlds with very different systems and goals.
From what I understand conducting for films is a precision art, back then you had to get used to click tracks and the like. If you didn't come from radio or television it was something you had to study. And even though much of WEST SIDE STORY has the benefit of being prerecorded, there is still much that had to be recorded traditionally. Leonard probably didn't think this technical and sometimes limiting art was worth learning for the few films he was involved in.