All right. I'll take a serious stab at this.

I don't think that the Bible is particularly "useful" as an instructor of
morals. The moral code in the Bible isn't profoundly different than that
embodied by the code of Hammurabi, other holy books, or, for the most part,
simply pragmatic common sense.

The power of the Bible is, and has always been, in its stories. All myths
survive because they speak to our fundamental human aspirations, hopes,
fears, and weaknesses.  Stories endure and spread because they speak to
these things in us.

The characters in the Bible are truly "mythic" characters, in the sense that
the great characters from other traditions are mythic. Great, but deeply
flawed.  The Mighty Man cursed by madness and doomed by a fatal weakness and
the betrayal of a woman finds form, in various incarnations, in Heracles, in
Cuchulain, in Samson, in Siegfried, even in Lancelot.  Clearly, somehow,
this figure comes not out of "history" but out of some very mysterious place
in the human mind. That character speaks to us.  When it pops up in a story,
that story tends to endure.

There are lots of other elements like this (and my knowledge of such things
is frankly limited). Pandora and her box -- Eve and her apple. The stealing
from the tree of knowledge and the punishment it wreaks -- the stealing of
the secret of fire from the gods. Stories of death and rebirth, of the great
Kings who betray their power. Sons who plot against their fathers and
daughters who sleep with their fathers. Stories of falling and redemption,
of transcendental salvation.

These things touch us. They move us, not as "facts" but as stories. It is
the mythic power of the Bible, not its factual, or historical, or even moral
power, in my opinion, that has caused it to endure.  These stories speak to
us in a deep way, in a way that bypasses reason and that touches something
that is fundamental in the human heart.

Moses sees the promised land before he dies, but he cannot cross over. Each
of us, in our time, will "look at the promised land" -- the "land" emodied
in our children, and in our deeds, and in all the changes we make in the
world. We see the "promise" embodied in our actions and our progeny. But we
cannot cross over. We can only look at what might be, and then die.

Whether there ever was a Moses, or a promise, or a "promised land" -- the
story has something to say to us. Rejecting the Bible as history, or
science, or even as a source of ethics, doesn't mean that we need reject it
for what its true source of power -- its power as Story.