While there is a slight argument to be made as to whether or not it was for "political expediency," those are indeed beliefs and traditions the LDS church has followed at one time or another (speaking to you from the heart and soul of Mormonism in Provo, UT):
Now as far as the Warren Jeffs, 12 year old bonking crowd, yes they're crazy, but no they merely started at the same root of the tree. No original-orthodoxy Mormons are left, they're dead. The rest -- at least much of the rest here in Utah -- seem to want to live their lives with blinders on about the past (and the outside world, help! I'm trapped in a bubble!). Go and ask your bishop about all of these things and he'll, a: sigh, and b: give you a well thought out, and historically accurate accounting of the church's somewhat malleable belief system.
Or as I do for the members of my ward who ask about these things, c) explain what Elder Oaks taught about the two channels of revelation and how understanding the history of the church ends up hinging on how well you and I can access knowledge from God.