I use bribes to weaken an enemy before going in with conventional invasion forces. Egypt is my favourite bribe target, because I hate their chariots full-stop. Romans bribing Romans are also an impressive exercise in money politics. I don't bribe enemies coming to relieve cities because I fight battles of annihilation, which means if they attack me, all the better. I am saved the trouble of an assault.

Bribing is a very useful tool, but I seriously doubt that any such usefulness will be possible in the 1.2 universe.