Role of minor factions

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

The role of minor factions is to have teir city captured easily .

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. 

In another post, Frogboy said that the role of minor factions is to guard the resource near them.

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

If you capture them, the city will never grow past level 1, but it won't cost food either IIRC. Also each indipendent city produces 1 metal or 1 crystal for free, yippie! (:()