I don't flush. Nothing is being "removed" from the plant. It's not evaporating, it's not breaking down, it's not leeching back into the soil. Flushing only deprives your girl of nutrients during her most productive weeks. Hydro growers only flush for 2 days and hydro gets 20% bigger yields on average. I don't think it's the grow method as much as it is the lesser flushing time.
Studies have shown no difference in harshness of smoke and people prefer the taste of unflushed bud.
So there you have it, it's just a tradition so engrained in the growing culture, most people can't shake it.