If Larkin makes this team better, he'll stay regardless. They didn't keep him on the roster just because there was room for him. They kept him because they believe he is ready for the NHL now. If he excels, he keeps his roster spot. It's someone else that will be moved elsewhere IMO. If the intention isn't to give him a legit shot at sticking with the team full time, he'd be playing on the top line with GR tomorrow night. Only way he gets sent down is if he struggles. And I think it is fantastic. Comparing him to Jurco really isn't fair. Two very different situations.
As for being bumped into the NHL over other guys, he is simply the most ready. Hockey isn't about tenure, it is about icing the best team you can. Every season a small few 18 and 19 year old kids break into the league. It is about time we have one that is good enough at that age. He already shown he can hang with NHL calibre talent. I see no reason why he can't learn his stuff with the Wings instead of GR. You save that for the kids who need to develop their game.