http://en.wikipedia.org/wiki/Ancient_astronauts
If ancient ones were visited by aliens, then these aliens were liars for they claimed the mantle of angels and spoke of the Throne of The Almighty One God.
At least with a god, you get a complete origin(1). With aliens, you just punt the "how did life originate?" question to a galaxy far, far away.
Its hella good fun for a few episodes of Stargate, but somewhat lacking as a cosmology.
(1) As long as you are prepared to deal with the "where/how did your god originate?" part.
What's more probable is that IF any ET exist they live in a galaxy far far away...too far to ever travel to ours.