It's an ok song for sure. Not only were there way better songs on Stewart's Bombs Away Dream Babies album, there was a better one featuring Nicks.
"Midnight Wind" wasn't released as a single - as far as I know - and if it were, it wasn't a hit. I'm not saying it should have been a hit, but to me, it was much more interesting than the radio-friendly, "Gold".
Not only was Stewart's part better than some of his other songs, Nicks actually shines. While I'm sure there was some direction of singing background / harmony on the song, I have to believe closer to the end she just ran with it.
I also believe it is songs like this that would eventually fry her vocal cords.