I actually popped on the git development irc channel a few months ago to make this suggestion. The two people I talked to (at least one of whom seemed to be a core developer, although I did not verify this) did not see how, if you're going to conflate the actions of creating a branch and checking it out, it makes more sense to do this as a flag on the primary action, branch creation.

They were nice enough to have an in-depth conversion with a complete newcomer, though, so it was not a negative experience overall. Maybe someone else could convince them :)