I got whiplash from shaking my head no to this deal...

I really don't get this trade for the Leafs perspective either. Bozak is needed at centre this year, and maybe even next year. And the Blues probably think they are better off this year with Shattenkirk than without him.

They would take a Marincin or Carrick, plus a pick or two, but not Bozak and Polak

Bozak also 30 and polak up there also.
Ive said it before I doubt the blues have much interest in bozak, and doubt the leafs would part with him if they are going for a playoff run

What's he needed for? The Leafs aren't viewed as a contender so hanging onto him blocks another player like Nylander the opportunity. Last year when the Leafs were tanking they played Nylander up the middle and he played well, this year they're trying to make the playoffs but that shouldn't stand in their way of continuing to rebuild.
St. Louis has taken a step back this year and is about to take another step back letting Shattenkirk walk for nothing. Maybe they want futures, maybe the market for Shattenkirk as a rental is weak, if the Leafs have interest, is it worth giving up a player like Kapenen or better for a player they could have on July 1?

what is that trade? polak and bozak are nowhere close to shattenkirk