If you look up melodica on Wikipedia, there are a couple of entries; one of which lists about 80 pop and rock songs in which the melodica was used. Songs from jazz and classical genres are there too.
One question is, can you play the melodica all night without burning people out?
After thinking about your follow-up question Kevin, and knowing a bit about composing and producing, I would say the draw to the melodica would have to be its unique sound. It is different than the accordion and harmonica. So if the melodica sound and vibe fit a song better, then the melodica would get the call.