I find the minors to be worth more to me alive than dead ..
The way I see it, since both races get credits from the trade routes, I might as well give it to a minor civ than another major civ that I am going to need to kill at some point. Plus if I keep the minors making money, that is just more cash they will have to trade. Basicly, I end up with that one trade route being worth 2x it's value, plus they are much more stable than trading with major civs. The only time I setup trade routes with a major civ, is if they are getting beat down by the other major civs, and I am not ready for a major war to knock someone's power down.
Plus you can get more money from selling outdated ships to minors than you do from decomission. I even give the minors some of my lesser planets sometimes just to keep them around. I always hate when the AI takes over ALL the minor civs!
Actually, I once "swapped" planets with a minor (give them a 10 class planet, declare war, take over the nice 15+ planet, declare peace). but even that is rare.