This makes more sense of Mr Palmer's role to me. I'd forgotten JA's over-riding theme.

And of course there is the comic role he plays as Mary Ellen pointed out.

Incidentally, I do disagree that Marianne and Elinor had to marry. Between the four of the Dashwoods, they have 500 pounds a year and can afford three servants, and there is no hint that Mrs Dashwood's seven thousand pounds would go to someone else on her death. Many people have lived out their lives on far less of an income. Nor does being an old maid in JA's time appear to have been the end of all happiness, eg JA's sister Cassandra appears to have lived a life with some meaning and occupation despite not being a first-class writer. Marianne and Elinor have their music and drawing respectively, both of which they seem to take very seriously, so I cannot see them lacking for intellectual interest if they do not marry.