Agger is a quality defender. Anyone who says otherwise is silly. I don't think there is a single centre-back on the planet who doesn't have a weakness. Vidic but he does often get exposed by pace. Pique is also exposed by pace sometimes but is great at reading the ball and distributing the ball Hummels is a great all round defender but isn't the greatest on the ball.
My point is that Agger is great at reading the game, interceptions, dealing with pacey strikers while having some issues with big strikers in the air. That doesn't stop him being a great defender. As is shown, we're still conceding aerial goals without him anyway.
There's nothing wrong with having a group of quality centre-backs who bring something different. Sakho is a great aggressive defender who reads the game really well, wins nearly everything in the air and his passing is quality. One weakness I've seen in the pass that he can be caught out of position on the odd occasion. However, I do think he is the future leader of our defence once he gets settled. Toure is the experienced guy with good pace, a great defensive brain, a great organiser and a great tackle. However, I don't think he's the greatest in the air either. (ala the 2nd Newcastle goal) Skrtel is a great fighter, fantastic in the tackle and often is there to make those last ditch tackles when needed. However, he's not a great distributer of the ball and isn't really capable of playing in a defence with a high line.
That is an extremely great selection of centre-backs to choose from. Whether you can keep all four happy is another story BUT for now, why not enjoy what we have and the depth we have in defence as I would be surprised if we have all four come the end of the january.