Agree 100%. Lex needs to have a shot a redemption, and the relationship is much richer if they were once friends and a big hope of Superman's is that they will be again.
While I do enjoy that element of the backstory, I kind of hope it's missing from the film.
Only because, it has been SO overdone IMO. It seems like every single villain has to be a friend/ally first and then 'turn to the dark side'.
I mean, just in the Superhero genre you've got: Norman Osbourne (Spiderman 1), Doctor Octavius (Spiderman 2), Harry Osbourne (Spiderman 3), Obadiah Stane (Iron Man), Magneto (X-Men, X-Men First Class), Loki (Thor), Henri Ducard (Batman Begins) Harvey Dent (The Dark Knight), Victor Von Doom (Fantastic Four), Hector Hammond (Green Lantern), Sinistro (Green Lantern (though granted, it's only in the last scene

)
Etc, etc, etc!
IMO, Lex should be a villain with no history of friendship with Clark/Superman. Their rivalry should be purely based on the fact that each of them respresents everything the other detests.
The Joker in TDK is the only one that springs to mind that's managed to give us a villain that's JUST a villain. And isn't that proof enough that it works very well that way?