despite this thread making it seem as if you are the only one who feels that way, there are actually many who agree with you (even i was reluctant to post

). not only have similar opinions been posted over the years, i've personally had many discussions with people (at home, while working at the
disney store, at WDW, as well as elsewhere) who feel that at the very least, there should be no additional charge for those living in the same household.
an additional adult charge may be industry standard, but many places also charge for children under 18yo - why is no one advocating that "industry standard"?
and i really doubt that an adult child traveling with their parents would cause more "wear and tear" on a room than a young child - we've seen lots of crayon and marker drawings on furniture and walls, fork gouges under tables, etc in WDW resorts: i doubt there are very many adult children who carry crayons and markers, or crawl under tables

.
that's another reason why we love having DVC - we can stay on WDW property, and have as many "adults" in our villa (as occupancy permits) without incurring additional fees.
heck, DVC members can even buy discounted APs for "family members living in same household", no matter what their age - at least one division of Disney seems to understand that many families remain close enough to want to vacation together, even once their "children" turn 18.