I thought the same to be honest but Sagna' stats are better than Richards or Zabaleta' and for that price it seems a good deal.Don't you have more than enough players in your team? RB there's Richard, Boateng and Zabaleta. I'll only say deal for Agger.
With his age as well and given the amount of competitions I'm in it's better to have choice. I'm thinking Boateng and Richards will eventually form the centre back pairing with Kolarov and Sagna down the flanks.
Agger is purely for competition. I only have Lescott who's a left footed centre back and he's been injured/banned a few times already this season meaning I've had to bring Kompany in who doesn't seem to play very well at all.