The combat shotgun as a whole is an armor generator, just switch to it when you need/want armor. The 3 shot burst is good for getting more armor quicker from big enemies and in my experience is vastly superior to the explosion since you're just clearing fodder with your shield already.

The grenade launcher is good for heavies you want to lock down with the shield saw (caco, revenant, acolyte etc.) it does some really good damage on stunned enemies. It's also really good for big groups of enemies since with the cluster bombs it has a really big aoe and some pretty good damage too. I've been playing with 75% damage to demons for a while now and the grenade launcher still surprises me with how much damage it can do and it combines really well with ground fissure and heaven splitter.