there's not any reason why you'd ever really need to.
Noakes M, et al. Comparison of isocaloric very low carbohydrate/high saturated fat and high carbohydrte/low saturated fat diets on body composition and cardiovascular risk. Nutr Metab (Lond). 2006 Jan 11;3:7.
That said, a lot of people find it easier to plan their meals by thinking in terms of a P+F meal or a P+C meal. If you are one such individual, then have at it.
I personally find it easier to remove carbs completely from certain meals when cutting, rather than having a tiny amount of carbs with each meal, so that's what I do, but it's more to do with how I like to eat rather than a need to separate carbs and fats.