From a money standpoint, Couture vs Lesnar is worth more money than Couture vs Fedor until the UFC hype machine gets behind it. Most mainstream people don't know anything about Fedor.
Couture doesn't stand much chance against Fedor anyway, he's never proven to be an elite striker (Chuck Liddell pwns Couture) and Fedor's ground game makes him look like Alexander Karelin in comparison to Couture.
And speaking of not deserving title shots, what did Randy do to deserve his shot? Get KTFO twice by Liddell and beat Mike Van Arsdale?
It's a weak division, and Lesnar has a real chance of winning the match, so he's as deserving as anyone else (except maybe Fabricio Werdum, but nobody would pay to see that).