I like McCarron for that very reason. He's last year's Mike Glennon. If we sign Josh McCown or Matt Moore (or nobody) - the league's going to know we're all in on one of the top guys in the Draft.
But with McCarron, there's legitimate reason to believe he might be the guy (like people thought with Glennon). But I don't think signing him precludes us from moving up.
And if our trading partners think it's possible we could back away from the table at any point, it makes negotiations easier. If they know we absolutely HAVE to get Darnold/Rosen/Mayfield/Allen - it puts us at a disadvantage.