During peak seasons, Resort ID's often times are checked on peak travel routes. This is to make sure that all the Disney Resort Guests who are paying top dollar get the service they've paid for, instead of not having enough room because of people abusing the system. (Not saying checking out a resort is abusing the system, but there are those out there who do abuse the system in other ways)
This is basically how its laid out:
One Day One Park tickets have access to the MK Ferry Boat, and the MK Express Monorail.
People with Park Hopper passes have access to the MK Ferryboat, MK Express Monorail, Epcot Monorail, and park-to-park bus transportation.
Disney Resort Guests have access to ALL Disney transportation including Theme Park and Resort access via Bus, Monorail, or Watercraft. In addition, they also have access to Disney Transportation to Downtown Disney and both water parks.
With the exception of Late Night at Pleasure Island, any Disney Resort Guest may travel to any Disney resort. After Midnight at PI, many drivers will check resort ID for two reasons... to make sure inebriated folks are travelling to the correct resort, and to protect the safety of guests by not allowing unwanted followers onto the bus.
90% of the time, these rules are not enforced. However, as said above, during peak times when abuse is high, and we need to make sure our Resort Guests are getting the service they've paid for, we will start to enforce them. It's those people who are paying for the service, afterall.