I'm not a Castro fan and I think the Cubs should trade him at the soonest opportunity, but that was pretty funny.
Trading one of their best players probably isn't a great idea, but ok.
He's two years removed from being one of the worst everyday players in the league. He does provide some value -- but that value is on a very, very slippery precipice. BJ Upton used to be a proven hitter, too, which is why the Braves signed him a few years ago, but no one bothered to look and see that his rate of swings on pitches outside the strike zone had been steadily climbing for like five years, and his walk and K rates were catching up to it. When looking at Castro, when your skill set is "BABIP-dependent singles hitter who runs really, really fast," your ceiling isn't terribly high. Given that the Cubs' problem is not scoring runs, but rather preventing them, Castro is probably the team's best asset for scoring pitching.
Again, he's not bad -- but he's not an Ubermensch, either. I don't think the team should be actively shopping him until they know what they have in Russell (and until they see if they can flip Baez now that he's been exposed), but if the right deal were to come along, absolutely, you flip him. Put another way, if a guy with a career 99 OPS+ is one of your team's "best players," then your team has issues.