You can be good at defence and great good forwards, or the opposite though and no one would care. Cole IS excellent at both, the problem with someone like Johnson or Sagna, is they can't defend, and worse than that they are lazy and simply VERY often don't bother coming all the way back after an attack, hoping they don't need to.
The amount of times the other team has the ball Sagna will rush back to the half way line(with the back line being another 20-30 yards deeper than that) and he'll stop and see if play get's dangerous or goes back up the field and he can attack again. So often we get caught out with him 20 yards behind the play as someone runs into the space behind him, then you get Kos running over to cover the massive gaping space, and then you get 3 defenders spread all over the place and overwhelmed, with the usual lack of any central midfielders tracking back either.
Sagna is very poor defensively, Johnson the same, Walker.... mistake prone, half a good season, half a very "meh" season. Beyond a couple goals pretty ineffective crossing and wrong choice making offensively. Walker has a HUGE amount of pace to make up for the errors in his game, however without the pace he'd be rubbish offensively and defensively, which to me says, rubbish. Gary Neville had zero pace and was just excellent defensively and very good getting forward.
Richards, is stupid powerful, a massive threat at set pieces, great defensively at set pieces, great timing getting forward, and strong defensively. Maybe the best tackler of all the right backs around, Ivanovic is excellent defensively, gets a few key goals, gets forward surprisingly well for a guy who is a CB and great at offensive/defensive set pieces.