Agree with everyone on this board. I think it will be a huge mistake if we let Agger go. Decent central defenders are hard to come by. In this modern game I don't believe we can have 2 traditional 'no nonesense' defenders. Skrtel and Carra have done well, but alas the time will come when Carra will have to go (can't believe I just said that!) And when that time comes I think we will be better equipped as an attacking force with someone at centreback who is comfortable / creative / able to distribute the ball.
Skrtel would be the perfect foil for Agger as the traditional no-nonsense defender. I think its very similar to the Ferdinand / Vidic partnership at Man U. We can't just have someone who can hoof the ball from centre back, we need someone who can start off play and move forward with confidence.