Mercutio, Kinsman to the Prince

I was thinking today about how often people accidentally lump Mercutio in with the Montagues, since he's a friend of Romeo and doesn't hold much love for Tybalt. That's of course not true, otherwise his "A plague on both your houses!" would make no sense.

Casual audiences forget that Mercutio is actually a kinsman to the Prince himself. It's pretty easy to forget, because ... tell me again how it plays into the story?

I was trying to figure this out. Mercutio needs to be neither Montague nor Capulet, that's clear. But Shakespeare could have just given him no affiliation. He doesn't have to be related to the Prince, does he?

What about Romeo's banishment? The Prince walks in to the bloodbath that was Tybalt/Mercutio/Romeo. He's told that his kinsmen Mercutio is dead, murdered by Tybalt. Tybalt, likewise, is murdered by Romeo. Romeo's gone.

The Prince, despite having promised execution for anybody that disturbs the streets again, decides on banishment for Romeo. How much do we think this decision has to do with the fact that it's Mercutio we're talking about? If Tybalt had murdered, say, Benvolio...then what? Does the Prince still call for banishment, or Romeo's head?

This is the only place I can think of where the relationship between Mercutio and the Prince might have played into the story. Is there another one?