The big argument for Sakho is that he's better in the air than any of the other lads, I kind of agree that him and Skrtel are probably too similar, in that they're not going to continuously try to read the game, but they will press players, hold players up and then put a foot in if needs be. There's nothing wrong with that, in most defensive partnerships you have one half of that. Someone to mop up and do the dirty work, it might seem less elegant but as far defensive play goes, it doesn't take a genius to see how crucial it is.
As for the "we had clean sheets with Agger", well yeah, for three games, we should probably look at the course of other seasons too, we've conceded goals regularly over the last few years with any combination of what we've got, including Super Dan, so three games that were won narrowly at the start of the season shouldn't really be a benchmark or sign of anything (have a look at the different combinations that played against Notts County, we were a joke). The only thing I'd take from those games is that while we were playing 442 we weren't any more vulnerable, so the so-called added assurance of a third centre back hasn't really shored things up for us, it's just taken a bit of pressure off the midfield.
The key, crucial flaw in our defensive system at the moment isn't really a lack of personnel, it's a defunct midfield that's offers little protection or dominance and a relatively poor outlet. We knew that ages ago.