Dpending on the laws, ideas and religious advisoryou have, omens may or may not well be worth it in the long run even at 50% chance.
For example, if your advisor, laws and ideas decrease the effect of a bad omen by 10%, then in the long run you will benefit from a 50% omen ((I usually always go for the research omen, except during war I boost the army and/or fleetwith omens; if you get for example +28% more research from a good omen but would get -22% from a bad omen, then I would go for it even with 50% chance of success). Then again, sometimes the oracles just should be hanged, like in a MP game where I had a stroke of nine bad omens in row before I got the first good omen (and that with a 60% chance)... Yet, the same thing in reverse prolly has happened numerous times, one just doesn't remember them.