Given that they won't have to pay for parking if they park in the MK lot, I'm not sure what the advantage would be. Drive all the way to VWL (which is very close to the MK lot), pay for valet parking - as opposed to parking for free in the MK parking lot? Personally, i'd rather take the bus from SSR. Much quicker and more efficient.
They won't be allowed to park in the self-parking lots all day at WL, CR, GF, or Poly. Those resorts issue 3-hour passes to folks who are coming to dine or shop, and there's a chance their car will be towed if they stay longer than 3 hours.
Many folks have reported that they were able to valet park and leave the car all day while they visit a theme park, but I don't know if I'd chance it. The valet lots must fill up at some point.