They technically do.
If you negotiate a Non-Aggression pact with them, you can enter their influence-area and buy some lesser magic items (early in the game) that you might not have access to (yet).
They also typically have a resource near them.
As for the rare unit, if you conquer them and have the proper research done, you can build on their special-unit tile and train that unit.

But in most cases, the role of minor factions' is... To die. >:3

I have one minor, Snathi city, and I'm glad I took it.  They had 2 magic items available, one of which I still can't get anywhere else (a ring that adds 2hp), but I haven't gone too far down the magic tree.  More importantly, it's still my only source of crystal.  There's no crystal mine, no anything near it except for ogres that I can't use, but it generates 1 crystal/turn. 

They don't seem to do a good job of it.... I usually can capture thier cities easily early in the game.