How Are Minor Races Useful Anymore?

While you can't trade with minor races in the trade window, you can send a freighter to their planet to establish a trade route.

Sending a freighter or a transport are the only two ways to interact with them, so they aren't very useful.  It has been mentioned that if you want a race to be less than normal, choose one and set its intelligence to the lowest level in the setup of the game.  I haven't tried this.  I just don't use minors.

I don’t play with minor races either since they are no longer useful. 

Personally id rather see some effort to resource management then a work done to improve minor races. If I want more interactive AI I just add another regular AI to the map

Ever since tech and resource trading with them was disabled I set minor races to none when starting a new game.

Same. I wish we can uplift them like in Stellaris and make them as our vassal.

I like that idea - I might even create some custom races with subpar abilities and stats for this very purpose. That's probably more fun than the way too predictable minors we've had.

IIRC, this is also what Dev answering the question about minor race a long ago. You could get many more races from mods/workshops so them minor races have no point in the game anymore. Just make a major one and downgrade them to act like minor.

Any idea if Minor races' behavior changes with increased Game or AI Difficulty?

Galaxy difficulty will boost them iirc, along with the pirate. They still not doing anything aside from being passive in their little planet. Not producing any fleet, putting up any offense so they're just there for Class 16 grab.