In short, the GOP primaries could decide who gets to be the Landon/Goldwater/McGovern/Mondale of 2012 if most things go right for Obama -- no foreign-policy blowups, no personal scandal, and some well-received reforms of the economic and political systems.
I still remember quite clearly back in 1992 a SNL skit parodying the Democratic primaries - who gets to be the guy to lose to George H.W. Bush in the fall? IN the skit, everyone was contending to not be the nominee.
Things change, and it is unwise to count your chickens too early...