Honestly if Smith plays, Zidlicky is more of a lateral move. You will still be stuck with a top pairing where Kronwall will be pulling the load (Ericsson or Zidlicky...not sure if the guy is top pairing quality at this point in his career). That would leave Smith and Ericsson at the bottom pairing, which I believe would be a complete train wreck.
I firmly believe that Quincey and Dekeyser have been the most consistent pairing this season so far, and will most liking stay paired up as the second pairing.
That leaves the option of "which pairing will suck less". Kronwall/Ericsson (Ericsson IMO does not below here) or Smith/Zidlicky? Do you risk a Smith/Ericsson as the bottom pairing?
This is where I think Marchenko would do wonders:
Kronwall/Zidlicky
Quincey/Dekeyser
Ericsson/Marchenko
Smith
Marchenko has shown that he has the ability to make other players look better: Take Smith for example. I believe he can do the same with Ericsson. Maybe not as well as Kronwall. But it will be enough to make sure the bottom pairing will not completely blow games. A Smith/Ericsson pairing, during the playoffs? You may as well start looking for a golf course now. That's assuming if you bolster your top pairing with Zids.
It's an upgrade for sure, but more on the lateral side because I think Ericsson will still be stuck on the top pairing...which we all know is OK, or face palm worthy. I also think he's holding back Kronwall from his offense.