I've found the pulse grenades useful from the beginning, sort of like mini power drainers. They're hard to kill enemies outright with (unless you corner them), but they knock out shields if you land them and someone's feet. Or knock out the shields of a clustered group of enemies simultaneously.

Also, unlike the other Forerunner weapons, they're unique and fictionally interesting. You can imagine prometheans busting into a Flood hive and disintegrating the growth off the walls with these. So why are we complaining about how the Forerunner weapons are too similar to existing guns, then turning around and complaining about the one that's new and interesting?