Do you use those large bottles of laundry detergent that sit on a shelf and have those neat no drip dispensers? There is no way to put the cap back on after measuring detergent and not make a big mess without rinsing the cup out first. Who wants to do that? It takes a lot of water to rinse it out and my sink and washer are not close in proximity. You could not rinse it and not replace the cap but you'll still have a mess in the cap that will dry out every time.
Here's the easy solution. Throw the cap in the washer with your clothes and you are finished with that mess. My cap fits upside down in the detergent compartment of my front loading washer which makes this solution even better since I don't have to find the cap among the wet clothes. It comes out so clean! Hope this helps someone!