I think he's that last little cherry on top of the marketing of this whole campaign. They've been taunting people with these little background shots of him, but I'm guessing he'll be part of that last blitz in the final week leading to release, along with the half dozen or so 60 second clips they'll undoubtedly release, as per routine, Marvel buildup.
I can't wait for his Hot Toys figure.
I really don't think they're going to put him in any mainstream marketing or trailers. I think he's being saved as a surprise for casual fans that don't know who he is. That way people aren't overwhelmed and it will happen organically.
He's still there for the die-hards as little teases that would be overlooked by mainstream, case in point would be the new poster. He's there, but only if you know it's him. Otherwise people would just see all Ultron drones and not think twice.