I believe that the only time one can use a Planet Buster safely, is when you are already elected Supreme Leader (Diplomatic victory) and one of the other factions refuses to accept that decision (perhaps due to "ahem" questionable integrity practices against them in the past). Then if you use a Planet Buster to eradicate that errant faction, the others are already commited to you.
In a past game, I thought the other factions would be sympathetic toward my retaliating likewise to a PB attack on my faction (surprisingly it was launched by the Peacekeepers) but they were not very supportive, turning on me like a pit bull on an a abusive trainer wearing a steak necklace.