I have observed, that this feature also can make the game easier in som circumstances.
4 Civs left, me, my ally the Korx, the Drath and the rampaging Yor.
The Yor declare war on the Korx, who instantly ask for help. I tried to get the Drath involved, but they would not declare war for anything I could afford.
After the Yor took 2 Korx planetes, they obviously got flagged as "on a conquest strategy" and the Drath threw themselves willingly in as on the long term it will secure their survival.
Now the Yor are getting creamed by the three of us and the whole thing will most likely end in an alliance victory of the korx, the drath and me.