Why have a nice camera if you aren't going to use it at such a photographic place--vacation at WDW? Unless photos are not that important to you, but then you probably wouldn't own a DSLR if photos weren't important.
I suppose it depends what they want to get out of it. For us, photos are our favorite souveniers and we have little kids, so I take a ton of pics. I do scrapbook and make photo books from each trip.
I definitely have had crappy photopass pics and have had trouble finding photogs in the parks at times.
Maybe, if the kids were older & we went to WDW more than once a year, I could consider leaving the big camera at home.
We do not carry the big old camera bag, tripod and all our lenses. We take a fanny pack style bag. It is compact and fits my DSLR w/ a lens attached + 2 additional lenses.
And I would think a small, gorilla pod set up on a table or railing could help steady the camera for fireworks photos and the like.
I look forward to the day that I don't have to bring diapers, wipes, etc to the parks, but I cannot envision a trip where I wouldn't bring my "good" camera.