The fact that you have to go back 50 years to find a Democrat SCOTUS appointee who ended up being more conservative than the president thought proves the point. As I posted a few months ago, only one of the eight Dem SCOTUS appointees since 1952 have ended up being more conservative than the president thought (Byron White, appointed by JFK in 1962), while 9 of the 18 GOP appointees since 1952 have been more liberal than the president thought (I’m not counting Roberts there, since his record was about what was expected until yesterday). That’s a 12.5% disappointment rate for Democrats and a 50% disappointment rate for Republicans. http://www.freerepublic.com/focus/bloggers/2849024/posts?page=108#108
As for naming governors or state judges instead of federal appelate judges, I don’t think that would improve our odds of picking conservatives who won’t move to the Dark Side. Earl Warren was a governor, and (as you mentioned) Souter was a state judge (although he did serve a while as a federal Circuit judge as well). It seems to me that picking someone with no federal judicial experience makes it likelier that we’d miss a red flag that would have come up during his service as a federal judge (or during the prior confirmation hearing). Then again, I think that U.S. Senators with legal backgrounds and who have served in the Judiciary Committee could make good SCOTUS nominees, and if Ted Cruz is elected to the Senate from Texas I’d like to see him in the Court some day (Sen. Mike Lee of UT could be a good candidate down the line as well).
I don't think it does, since in the modern era, the RATs have had far less opportunities than Republicans to name SCOTUS judges. They haven't even named a CJ in 60 years. If the RATs president were appointed as many judges as the GOP, there'd be a far great opportunity for slip-ups. It's:
Obama - 2
Clinton - 2
Carter - 0
Johnson - 2
JFK - 2
vs.
GWB - 2 (1 CJ)
GHWB - 2
Reagan - 4 (1 CJ)
Ford - 1
Nixon - 4 (1 CJ)
Eisenhower - 5
If the RATs had 3 times as many judges, I think they'd be bound to have far more slip-ups than Byron White. Plus, alot of the GOP duds didn't become defacto Dems (Souter is only example of that since the mid 70s), but they were right-of-center judges who voted conservative far less than expected (O'Connor, etc.)
I think the biggest problem is the "evolving justice" issue. You're right, Roberts was a pretty reliable conservative for the last 6 years, but if this most recent session is a sign of things to come, we're in trouble. We can't afford to have Roberts go wobbly on us. And I've never seen a RAT justice go wobbly or "evolve" into a conservative. White was a fairly moderate right-of-center justice throughout his tenure on the court. Blackmun, on the other hand, started out as a "Minnesota twin" of Warren Burger, and ended up as the court's most liberal member when he retired in 1994.
One thing I will say is a bad factor on how to choose a judge: the concept that it doesn't matter what their personal views or record is, as long as their "judicial philosophy" is a "strict constructionist" or "originalist" or whatever they're calling it now (I never heard of this term "originalist" until a bunch of freepers started demanding GWB appoint one). Most of the "strict constructionists" that Nixon appointed came back to bite him in the butt big time. I want a lifelong, principled CONSERVATIVE, not some judge whose personal views are unknown but assures the President he is a "strict constructionists" Hell, I think Souter promised Bush that too. Anyone would say that if it meant a lifetime slot on SCOTUS.