As one of my esteemed colleagues wrote about ten minutes ago when we discussed this move, any time you can get the third cornerback on one of the league's least effective secondaries to replace Brent Grimes, you do it.
Terrence Johnson is fast. Johnson is fairly young (26 years old). Johnson has experience playing cornerback for an NFL team. Outside of that, you'd be hard-pressed to mount a strong case for this signing, being reported by the reliable Adam Caplan as imminent.
Johnson was an undrafted free agent who bounced around practice squads during his first season in the NFL. He wound up being pressed into slot duty for a while with the Colts' woeful secondary in 2011 before landing on injured reserve in late October, where he showed a penchant for being able to catch up with and tackle guys but not necessarily to stop them from catching the ball.
It's impossible to know to what degree he struggled thanks to the fact that the entire defense was putrid, and standing out in that mess would have taken a level of talent few NFL corners have. But looking at this signing as anything more than the Falcons grabbing a young guy with a little experience in the slot is a mistake, because that's exactly what this is. They clearly think he has some upside, yes, but
If you didn't think the Falcons were rolling with Chris Owens and Dominique Franks out there for the moment, then I'm afraid you were mistaken. Maybe Johnson will work out well down the road, but for now, that's the way it is. Let's hope the in-house talent is ready to contribute.