Well, first of all, tweener is not a word I would use to describe Jason Allen. The word tweener suggests that he is a guy that is caught in between two positions, hence the word TWEENer.
That's not Jason Allen. Jason Allen is a VERSATILE player that can play BOTH cornerback and safety. To me, that means he is not a tweener. He's a lot like Michael Huff, with the exception that Jason Allen has more experience as a cornerback.
Jason Allen gives the Dolphins a lot of flexibility in the secondary. The first thing they will do is identify his primary position.
If they decide that he will play cornerback, then he gives us the flexibility to move Travis Daniels inside to the "star" position that Reggie Howard played last year. His versatility and ability to play the "star" position himself will allow the Dolphins to also play Travis Daniels at the "money" position with Will Poole lining up on the outside.
If they decide that he will play safety, then he gives the Dolphins the added advantage of having a safety that can line up man to man on a slot receiver. His versatile ability will also allow the Dolphins to move him to the "star" position in the nickle packages while bringing in a nickle safety like Renaldo Hill.
His versatility has no downside in this defense.