Those first areas where you come across those cherub things, the grenades are useful there...
Just before that area, when you have to clear out the gasses, you could do away with the imp, zombie and revenant with the grenades... The zombies and maggots are usually in place when you enter, so I spam the grenades into corners to get them before looking around too much... drop them down elevator shafts for cheap kills, too..
Not sure I'd want to lose them for flares... then again, I'm enjoying the game as is:)