I support both sides of this argument. When I am in late flowering I like to flush lots of water through my soil, and then feet a dilute nutrient feed at the end. I find that this hydrates the plant with the initial flus by wetting the soil thoroughly and so activating all the roots. Then once the soil is wetted properly, a feed of just a few litres of diluted flowering nutes to leave some nutrients in the soil. If you were to provide this level of nutrients to the plant over the whole fush beforehand then it would be too diluted to be beneficial. If you did not flush, but used your concentrated nutrients to provide the plant with all of its water then this may be fine, but the problem is that when you have soil like mine, that would cause too much of a build up of salts in the soil. For me however, the flushing process is not to get rid of nutrients from the plant, it is primarily to thoroughly hydrate the roots of the plant, giving it a good drink of plain water!