Doing it this way seems to make the most sense for the President (in that it saves a confirmation hearing), plus it seems like he wanted Roberts for Chief Justice all along. He just never expected O'Connor to retire before Rehnquist died.
"Roe v. Wade is the settled law of the land. It's a little more than settled. It was reaffirmed in the face of a challenge that it should be overruled in the Casey decision. Accordingly, it's the settled law of the land. There's nothing in my personal views that would prevent me from fully and faithfully applying that precedent, as well as Casey." — Roberts, during the confirmation hearing, when asked for his own views on Roe v. Wade.
(
http://www.sfgate.com/cgi-bin/articl...w090602D23.DTL)
I also think it's too early to declare Roe v. Wade "doomed," although I'm not surprised that you used that wording.