Well, Overseer is nice, but the M12 crappy lord (Automaton-something?) could actually fill in the role (or something else could come in, of course), and Ornithopter is explosive, and all, but... well, on the end of the day it's still an 0/2, I was never a big fan of it, the way I see it, most decks I like play good cards, and if they come together fine the deck is great - if not, hey, they're still good cards; Tempered Steel plays a bunch of crappy cards that, when together, make the deck great - when they do not come, they're... crappy cards. I'd rather move away from an 0/2 into something slightly better, considering this.
PS: If decks keep playing less and less removal, U/G Infect might be appealing post-rotation...
PPS: If blue zombies are supposed to be Frankenstein monsters, does that mean Lightning Bolt IS coming back, only in Innistrad instead of M12? Shock's flavorful enough, though... Point is, if Lightning Bolt bites it, people will play even less removal, since maindecking Shock as removal is kind of a bummer.