I am with you, OP.
Growing up, and even now, the whole point to a vacation was to squeeze in as much FUN during the day, no matter where you were, and the hotel was just a place to sleep at night (and maybe swim in the afternoon).
I shared a bed with my little brother until I hit puberty, then I slept with my mom(single parent). Yeah, that meant the 7 year old got a bed to himself, but oh well.
We have always squeezed all 5 of us into a room at WDW, usually the trundle-bed rooms at POR. DH and I in one bed, two oldest DDs in the other, and youngest DD in the trundle.
I have always shared a room with my BFF when we take weekend shopping trips.
Also, when I went with DD19's band in her junior year of high school to SoCal and
Disneyland, all the chaperones had to buddy up on rooms. I was lucky that one of the chaperones was DD's best friend's mom, so I didn't get stuck with a complete stranger, but I still shared a room for a week with someone I was not related too. We did just fine, had fun even!
I just cannot see wasting the money on a room for one person, who is related to the other people, when there is room for everyone to stay in one room. Save the money and do something fun with it, IMHO!