Not just fit...fit is king and seatback is a big part of fit because your fore aft position relative to the BB is kind of a big deal for comfort and speed.
As to the saddle cradle as you put it curving back..yes, that is what a setback seat post is...when the centerline of the clamp is well behind the centerline of the seat post itself. For the record, a Thomson post with its bend rearward is one of the least setback posts out there...only 16mm...because the clamp Thomson uses has no offset and the bend is only about 10 degrees or so. Not to tell Thomson their business but they limit their market by not offering a setback post in the 20-25mm range. They do this so they don't have to tool another clamp and their std clamp has enough angle range to work with the slight bend in the seat post.
But...for many riders a 16mm offset is not enough with the contemporary shift toward more upright seat tube angles which promote shorter chainstays and shorter wheelbase. I prefer a carbon post at the end of the day anyway so no big deal to me. Thomson makes a good Al post tho.

There is nothing relaxed with his setup. It is a race geometry setup with some minor compromises to fit a wide variety of riders. His stem appears to be 120mm -17 slammed.


http://cdn1.media.cyclingnews.future...l_view_600.jpg

His setup isn't relaxed, that frame has a slack STA.

From Cervelo:
"To save a few more grams of bike weight, the R5ca is designed to use a zero offset post. Accordingly, with the saddle setback fixed, the seat tube angle has been decreased to achieve a 25mm rearward offset at the saddle height. This means the R5ca with zero offset post fits exactly the same as any road bike with 73 degree seat tube angle and traditional 25mm offset post."

Hilarious. You are right of course. Gotta love the 41. :)

show me some pros with flites slammed forward on straight posts on "crit" frames, and ill be surprised.

so no matter what your set up, knee over spindle is the ultimate goal then?

So Danielson composes the entire pro peleton? You'd think he'd win more often.

Here's a more successful rider:

http://cdn4.media.cyclingnews.future...d_drop_600.jpg

and another:

http://cdn2.media.cyclingnews.future...g_6155_600.jpg

Of course this guy was stripped of a bunch of results

kops, no. kbps.

No. It might be used as a starting point (or an endpoint by fitters who don't know what they're doing). Here's one viewpoint:

http://sheldonbrown.com/kops.html