I mean . . . Grass is the most common plant on Earth. Nearly every fire that ever happens involves grass burning in some capacity. I passed a house-fire like a year ago and both the bushes and grass were burning.
So should it burn? Sure. Most fires in Cataclysm are intentionally set or involve a building burning down. Both of those situations imply a strong, healthy fire, not a naturally occuring spark or ember that may or may not ignite healthy, living plants.
If you take issue with grass burning you should have an issue with you setting a living tree or bush on fire.
I think fire is good the way it is, but I think there should be a stronger variable as to the speed with which it spreads based on the material being ignited.