The only thing the oil seed radish has going for it is that it is really cheap compared to buying liquid or solid fertilizer and applying it 75l of seed hectare in fs 17 and probably similar in fs 19 so if you are tight on money and willing to spend a bit more time then you can save a few bucks. It is almost a free fertilization step it barely uses seed compared to normal crops so it is fairly cost efficient. If you use them once you seed over them with a normal crop you can fert again.