It’s a good thing for supporters of traditional marriage that the Court granted review both in the Prop 8 case and in one of the Defense of Marriage Act cases.
For starters, the rulings below were both adverse to traditional marriage, so a denial of review in either case would have left the adverse ruling in place.
More broadly, the challengers to DOMA have been employing a stealth strategy, invoking phony and confused federalism concerns (see point 1 here) and trying to pretend that DOMA can coherently be invalidated on grounds that wouldn’t dictate a broader constitutional right to same-sex marriage. The fact that the Prop 8 case is being considered together with the DOMA challenge ought to expose the stealth: Same-sex marriage advocates can’t plausibly be heard to maintain in their attack on DOMA that marriage is the traditional preserve of the states at the same time that they’re arguing in the Prop 8 case that the federal Constitution compels states to require same-sex marriage. Nor (as I will explain more fully) is there any coherent way to strike down DOMA that wouldn’t also require striking down Prop 8 and other state laws preserving traditional marriage.
To be clear, I am certainly not predicting that supporters of traditional marriage will be victorious in either or both cases (although I strongly believe they should win both). But their prospects of prevailing in both ought to be enhanced by having the cases considered together.