The Jets are not the kind of team that will stretch the field with great deep receivers. Percy Harvin is certainly not a great vertical threat. With the addition of him, and with Michael Vick stepping into the offense, the Jets will likely to some extent substitute for that with their run game. These additions potentially force the defense to cover the entire field to respect the run game.
What do I mean?
It's third and one, and the Bills are loading the box. They're committing their resources up the middle, but the edge is exposed. Harvin is lined up in the backfield and going to take it outside.
Because Buffalo is sending so many players to stop the inside run, they all get bottled up as Harvin takes it outside.
Willie Colon misses his block. Harvin gains 6 and a first down, a succesful play. If Colon doesn't miss his block, Harvin might be off to the races.
The Jets haven't been able to punish teams who have sold out against the run. The dimension Harvin adds makes that more possible.
Now let's take a look at another play where Harvin's presence was felt.
It's a simple read option to the right. Vick is going to read the unblocked defensive end and decide whether to give it to Harvin or keep it himself.
In this case the defensive end bites on the fake and chases Harvin. This leaves the starting position he vacated open for Vick to run and pick up 8 yards.
The Jets aren't doing anything out of the ordinary here, but the infusion of speed into the lineup both Vick and Harvin bring make this play more dangerous. The defender has to make a decision quicker than he normally would. Harvin is so fast that he'll be past the defender who waits too long to take him and could be off to the races. Overpursue Harvin, and Vick will hit the hole before you can recover.
Ultimately what this does is force Chris Ivory to face less loaded boxes. The threat of Harvin leaves him more room inside to operate.
The Jets are probably going to have to become a heavy run team. Let's face it. They are going to be the less talented team on the field in most if not all their remaining games. They also need to hide their defense. Running the ball will limit the number of series the defense sees and the number of stops it will have to get. The Jets also have to leave Vick exposed as little as possible. He's liable to throw terrible interceptions, and this offensive line leaves him vulnerable.
The Jets now have three guys on their team, however, who can break a big run. Now this team is a threat to do so both inside and outside on any given play. If you can't stretch the field vertically, stretching it horizontally can make up for it at least to a certain extent.