Many attractions involve a preshow or a wait in a 'holding area' for 10-15 minutes or more. Those are often shows that have guests 'collect' in a prehow area while another 'set' of guests is in the show. Once the first guests leave the theater, the doors open for the next set of guests who are waiting in the preshow area.
Having a DAS (Disability Access Service) card will not prevent you from standing in those situations. Most of the areas have a very few or no seats at all and you have to wait in the preshow area to get into the show.
Some examples of places where you would have to stand - DAS or not would be:
Studio
Great Movie Ride
American Idol Experience
Muppet Vision 3D
Voyage of the Little Mermaid
One Man's Dream (This is a walking tour)
Twilight Zone Tower of Terror
Magic Kingdom
Railroad - there is not a preshow, but the train has to come to the station for you to board it.
Tiki Room
Hall of Presidents
Haunted Mansion
Country Bear Jamboree
Mickey's Philharmagic
Monster's Inc. Laugh Floor
Stitch's Great Escape
Enchanted Tales With Belle - the first part of the experience is about a 2 minute introduction to the story while standing in Belle's father's workshop. The second part is standing in the wardrobe room for about 5 minutes while roles are assigned for play parts. There is no place to sit for either of these. Guests are seated on backless padded benches for the remainder of the show.
Epcot
Ellen's Energy Adventure
Mission Space
Test Track
Circle of Life (Movie at The Land)
Soarin'
American Adventure
Reflections of China (a 14 minute movie that has no seats, just rails to lean against)
O Canada (a 14 minute move that has no seats, just rails to lean against
Animal Kingdom
It's Tough to Be a Bug
Festival of the Lion King (you will need to wait for the next show)
Wildlife Express Train (you will need to wait for the next train - they leave every 5-7 minutes)
Flights of Wonder (there may not be a place to sit when waiting for the next show)
Finding Nemo (you will be standing during your wait for the next show)
Having a wheelchair or
ECV would give you a place to sit during those kinds of attractions and would also help you cover the distances for a WDW visit. Many people don't think about how much they walk in a day at WDW, but the average is at least 3 miles per day.
Many of the lines are very long distance to walk from the entrance to the actual boarding area. For example, Soarin is about 1/4 mile from the entrance to the line until the boarding area. There is an equal distance from the point you complete your flight until you get back out of the ride.
A DAS would not usually shorten the distance and the DAS is only used in lines and does nothing to help you get from place to place.