Krupe is relevant to WSS, aside from the fact that it adds some humor to the tension. Though it doesn't directly affect the intense action and tension of the second act, it serves as important commentary on the situation--the police/adults are completely clueless when it comes to handling these "juvenile deliquents"--and they're well aware. This completely coincides with the last visual pose of the musical--the gangs have walked out, and all the adults stand on the stage, looking at each other, completely clueless has to handle any of this. It's an important theme of WSS, and an important underlying in all the action that goes on.
I want to disagree with Sit Down You're Rocking the Boat, but thinking about it, you are right.
Feed the Birds has very important plot relevance! For the movie, Mary instills the idea of feeding the birds, so Michael won't give his toppins to the bank because he wants to feed the birds, that's what starts the whole problem at the bank and results in Mr. Banks' almost firing, and eventual character change.
I'll Share It With You and Who Do You Love I Hope don't seem to serve that big a function in Annie Get Your Gun, aside to give those characters something to sing, and to be a bitch to the musicians.

Old fashioned wedding kind of feels the same way, since as soon as the song is over, they decide not to get married. Moonshine Lullaby is my favorite song, but really serves no plot relevance. I don't think Sun in the morning is that important either, except for a huge dance break.
My favorite song in Guys and Dolls may not be that important either--More I Cannot Wish You. i guess it serves some purpose, encouraging Sarah to accept/love Sky.
I'm not sure that Ascot Gavrotte is essential in MFL. I guess it establishes the class distinctions and the world Eliza is trying to get into, but it doesn't need to be described in song. Plus, I don't like it.
Though I really like it, Wishing You Were Somehow Here Again isn't essential to Phantom. I mean, we know Christine misses her father. It's nice to get her to get the grave for Wandering Child, but she didn't need to sing. It's mostly a show-off song for Christine.
That's enough for now. There's definitely more I can name.
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli