The ratio-rite is already a fantastic product, and the lid is a simple improvement that is worth making. Not only does it keep dirt out of the cup, it also keeps any remaining oil inside it. There's no reason not to have one.
This lid doesn't really seal tight, given the way the 'Ratio Rite Measuring Cup' is designed with the raised lip to prevent dripping. Therefore, the lid doesn't seall 100% tight...but will keep dirt and crap out of the cup, so it is somewhat functional.