Now they may not be 'points efficient' but they work great. Don't get fooled by their points into thinking they are a durable unit on their own, cause they are not. 3 squads hold my rear and dig in to cover and my aegis, the other 2 are in vedettas to grab objectives and to make a play and counter attack my enemy. They are guardsmen and even vets with carapace armor are no more than expendable pieces to use to reach victory. You aren't and if you are trying then you are playing the wrong army. Now don't for a second think because you threw carapace armor on you are now a space marine. You can still bring the numbers that the guard need but now you have troops that can hang with Dire avengers and fire warriors and the other carapace armored equivalents. You do have to take veterans to get access to carapace and it is a 30 point upgrade, but they really aren't expensive to begin with and I can fit 50 vets in my army with plenty left over for other choices. This really jumps your guardsman's survivability in a big way. But for a lot of things, you get a chance to live. Granted they fire the riptide big blast and ignore cover you die, but so does everyone else and then they are wasting that on guardsmen. Against Tau this makes a big difference, as the SMS is AP5 and so is their small arms.

Eldar is more limited but they can do it with psychic powers, tau have markerlights and SMS. So the big boys in the room are eldar and tau, and both can ignore cover in their own ways. This also comes even more into play against the armies that have abilities to ignore cover. The vast majority of small arms in the game have AP5, so now instead of just dying, you get an armor save. Boltguns, pulse rifles, flamers, shootas, etc. The big thing were this comes in is against small arms fire. Now yes there are helldrakes and other weapons that beat the 4+, but they autokill most things anyways so nothing you can do about that. With the rise of armies that can just ignore cover, the 4+ save means that many of those weapons that would just kill a regular guardsmen outright, now you have a chance of surviving. However the current meta makes it very useful and worth its points completely and here is why. Now I have visited some forums and for whatever reason, the general consensus among IG players is that carapace armor is worthless. Its small but they at least have a chance. I personally love carapace armor, and like the idea of my elite shock troops going into the fight and standing a small chance of surviving. The standard issue cardboard seen as good enough, but for some there is the ability to take carapace armor. Now the imperial guard is not known for the quality of the armor it provides to guardsmen.

Warhammer 40k imperial guard armor